Fix use-after-free in source_cache::get_source_lines

Fix use-after-free in source_cache::get_source_lines

Commit ab42892fb7d2 ("Fix vertical scrolling of TUI source window")
introduced a use-after-free in source_cache::get_source_lines.

At the beginning of the method, we get the fullname of the symtab:

    const char *fullname = symtab_to_fullname (s);

fullname points to the string owned by the symtab (s.fullname).  When we
later do

    scoped_fd desc = open_source_file (s);

s.fullname gets reallocated (even though the string contents may not
change).  The fullname local variable now points to freed memory.

To avoid it, refresh the value of fullname after calling
open_source_file.

Here is the ASan report:

$ ./gdb -nx --data-directory=data-directory ./a.out
(gdb) start
Temporary breakpoint 1 at 0x1130: file test.cpp, line 12.
Starting program: /home/simark/build/binutils-gdb/gdb/a.out

Temporary breakpoint 1, main () at test.cpp:12
=================================================================
==26068==ERROR: AddressSanitizer: heap-use-after-free on address 0x6210003d4100 at pc 0x7fed89a34681 bp 0x7ffd8d185d80 sp 0x7ffd8d185528
READ of size 2 at 0x6210003d4100 thread T0
    #0 0x7fed89a34680 in __interceptor_strlen /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:301
    #1 0x55b6edf6c2f7 in std::char_traits<char>::length(char const*) /usr/include/c++/8.2.1/bits/char_traits.h:320
    #2 0x55b6edf6c9b2 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&) /usr/include/c++/8.2.1/bits/basic_string.h:516
    #3 0x55b6ef09121b in source_cache::get_source_lines(symtab*, int, int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*) /home/simark/src/binutils-gdb/gdb/source-cache.c:214
    #4 0x55b6ef0a15cb in print_source_lines_base /home/simark/src/binutils-gdb/gdb/source.c:1340
    #5 0x55b6ef0a2045 in print_source_lines(symtab*, int, int, enum_flags<print_source_lines_flag>) /home/simark/src/binutils-gdb/gdb/source.c:1415
    #6 0x55b6ef112c87 in print_frame_info(frame_info*, int, print_what, int, int) /home/simark/src/binutils-gdb/gdb/stack.c:914
    #7 0x55b6ef10e90d in print_stack_frame(frame_info*, int, print_what, int) /home/simark/src/binutils-gdb/gdb/stack.c:180
    #8 0x55b6ee9592f8 in print_stop_location /home/simark/src/binutils-gdb/gdb/infrun.c:7853
    #9 0x55b6ee95948f in print_stop_event(ui_out*) /home/simark/src/binutils-gdb/gdb/infrun.c:7870
    #10 0x55b6ef34b962 in tui_on_normal_stop /home/simark/src/binutils-gdb/gdb/tui/tui-interp.c:98
    #11 0x55b6ee01a14d in std::_Function_handler<void (bpstats*, int), void (*)(bpstats*, int)>::_M_invoke(std::_Any_data const&, bpstats*&&, int&&) /usr/include/c++/8.2.1/bits/std_function.h:297
    #12 0x55b6ee965415 in std::function<void (bpstats*, int)>::operator()(bpstats*, int) const /usr/include/c++/8.2.1/bits/std_function.h:687
    #13 0x55b6ee962f1b in gdb::observers::observable<bpstats*, int>::notify(bpstats*, int) const /home/simark/src/binutils-gdb/gdb/common/observable.h:106
    #14 0x55b6ee95a6e7 in normal_stop() /home/simark/src/binutils-gdb/gdb/infrun.c:8142
    #15 0x55b6ee93f236 in fetch_inferior_event(void*) /home/simark/src/binutils-gdb/gdb/infrun.c:3782
    #16 0x55b6ee8f2641 in inferior_event_handler(inferior_event_type, void*) /home/simark/src/binutils-gdb/gdb/inf-loop.c:43
    #17 0x55b6eea2a1f0 in handle_target_event /home/simark/src/binutils-gdb/gdb/linux-nat.c:4358
    #18 0x55b6ee7045f1 in handle_file_event /home/simark/src/binutils-gdb/gdb/event-loop.c:733
    #19 0x55b6ee704e89 in gdb_wait_for_event /home/simark/src/binutils-gdb/gdb/event-loop.c:859
    #20 0x55b6ee7027b5 in gdb_do_one_event() /home/simark/src/binutils-gdb/gdb/event-loop.c:322
    #21 0x55b6ee702907 in start_event_loop() /home/simark/src/binutils-gdb/gdb/event-loop.c:371
    #22 0x55b6eeadfc16 in captured_command_loop /home/simark/src/binutils-gdb/gdb/main.c:331
    #23 0x55b6eeae2ef9 in captured_main /home/simark/src/binutils-gdb/gdb/main.c:1174
    #24 0x55b6eeae30c2 in gdb_main(captured_main_args*) /home/simark/src/binutils-gdb/gdb/main.c:1190
    #25 0x55b6edf4fa89 in main /home/simark/src/binutils-gdb/gdb/gdb.c:32
    #26 0x7fed88ad8222 in __libc_start_main (/usr/lib/libc.so.6+0x24222)
    #27 0x55b6edf4f86d in _start (/home/simark/build/binutils-gdb/gdb/gdb+0x197186d)

0x6210003d4100 is located 0 bytes inside of 4096-byte region [0x6210003d4100,0x6210003d5100)
freed by thread T0 here:
    #0 0x7fed89a8ac19 in __interceptor_free /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cc:66
    #1 0x55b6edfe12df in xfree<char> /home/simark/src/binutils-gdb/gdb/common/common-utils.h:60
    #2 0x55b6edfea675 in gdb::xfree_deleter<char>::operator()(char*) const /home/simark/src/binutils-gdb/gdb/common/gdb_unique_ptr.h:34
    #3 0x55b6edfe532c in std::unique_ptr<char, gdb::xfree_deleter<char> >::reset(char*) /usr/include/c++/8.2.1/bits/unique_ptr.h:382
    #4 0x55b6edfe7329 in std::unique_ptr<char, gdb::xfree_deleter<char> >::operator=(std::unique_ptr<char, gdb::xfree_deleter<char> >&&) /usr/include/c++/8.2.1/bits/unique_ptr.h:289
    #5 0x55b6ef09ec2b in find_and_open_source(char const*, char const*, std::unique_ptr<char, gdb::xfree_deleter<char> >*) /home/simark/src/binutils-gdb/gdb/source.c:990
    #6 0x55b6ef09f56a in open_source_file(symtab*) /home/simark/src/binutils-gdb/gdb/source.c:1069
    #7 0x55b6ef090f78 in source_cache::get_source_lines(symtab*, int, int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*) /home/simark/src/binutils-gdb/gdb/source-cache.c:205
    #8 0x55b6ef0a15cb in print_source_lines_base /home/simark/src/binutils-gdb/gdb/source.c:1340
    #9 0x55b6ef0a2045 in print_source_lines(symtab*, int, int, enum_flags<print_source_lines_flag>) /home/simark/src/binutils-gdb/gdb/source.c:1415
    #10 0x55b6ef112c87 in print_frame_info(frame_info*, int, print_what, int, int) /home/simark/src/binutils-gdb/gdb/stack.c:914
    #11 0x55b6ef10e90d in print_stack_frame(frame_info*, int, print_what, int) /home/simark/src/binutils-gdb/gdb/stack.c:180
    #12 0x55b6ee9592f8 in print_stop_location /home/simark/src/binutils-gdb/gdb/infrun.c:7853
    #13 0x55b6ee95948f in print_stop_event(ui_out*) /home/simark/src/binutils-gdb/gdb/infrun.c:7870
    #14 0x55b6ef34b962 in tui_on_normal_stop /home/simark/src/binutils-gdb/gdb/tui/tui-interp.c:98
    #15 0x55b6ee01a14d in std::_Function_handler<void (bpstats*, int), void (*)(bpstats*, int)>::_M_invoke(std::_Any_data const&, bpstats*&&, int&&) /usr/include/c++/8.2.1/bits/std_function.h:297
    #16 0x55b6ee965415 in std::function<void (bpstats*, int)>::operator()(bpstats*, int) const /usr/include/c++/8.2.1/bits/std_function.h:687
    #17 0x55b6ee962f1b in gdb::observers::observable<bpstats*, int>::notify(bpstats*, int) const /home/simark/src/binutils-gdb/gdb/common/observable.h:106
    #18 0x55b6ee95a6e7 in normal_stop() /home/simark/src/binutils-gdb/gdb/infrun.c:8142
    #19 0x55b6ee93f236 in fetch_inferior_event(void*) /home/simark/src/binutils-gdb/gdb/infrun.c:3782
    #20 0x55b6ee8f2641 in inferior_event_handler(inferior_event_type, void*) /home/simark/src/binutils-gdb/gdb/inf-loop.c:43
    #21 0x55b6eea2a1f0 in handle_target_event /home/simark/src/binutils-gdb/gdb/linux-nat.c:4358
    #22 0x55b6ee7045f1 in handle_file_event /home/simark/src/binutils-gdb/gdb/event-loop.c:733
    #23 0x55b6ee704e89 in gdb_wait_for_event /home/simark/src/binutils-gdb/gdb/event-loop.c:859
    #24 0x55b6ee7027b5 in gdb_do_one_event() /home/simark/src/binutils-gdb/gdb/event-loop.c:322
    #25 0x55b6ee702907 in start_event_loop() /home/simark/src/binutils-gdb/gdb/event-loop.c:371
    #26 0x55b6eeadfc16 in captured_command_loop /home/simark/src/binutils-gdb/gdb/main.c:331
    #27 0x55b6eeae2ef9 in captured_main /home/simark/src/binutils-gdb/gdb/main.c:1174
    #28 0x55b6eeae30c2 in gdb_main(captured_main_args*) /home/simark/src/binutils-gdb/gdb/main.c:1190
    #29 0x55b6edf4fa89 in main /home/simark/src/binutils-gdb/gdb/gdb.c:32

previously allocated by thread T0 here:
    #0 0x7fed89a8b019 in __interceptor_malloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cc:86
    #1 0x7fed88af983f in realpath@@GLIBC_2.3 (/usr/lib/libc.so.6+0x4583f)
    #2 0x7fed899dbbbc in __interceptor_canonicalize_file_name /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:3297
    #3 0x55b6ee376a03 in gdb_realpath(char const*) /home/simark/src/binutils-gdb/gdb/common/pathstuff.c:72
    #4 0x55b6ef09ec12 in find_and_open_source(char const*, char const*, std::unique_ptr<char, gdb::xfree_deleter<char> >*) /home/simark/src/binutils-gdb/gdb/source.c:990
    #5 0x55b6ef09f56a in open_source_file(symtab*) /home/simark/src/binutils-gdb/gdb/source.c:1069
    #6 0x55b6ef0a0f12 in print_source_lines_base /home/simark/src/binutils-gdb/gdb/source.c:1270
    #7 0x55b6ef0a2045 in print_source_lines(symtab*, int, int, enum_flags<print_source_lines_flag>) /home/simark/src/binutils-gdb/gdb/source.c:1415
    #8 0x55b6ef112c87 in print_frame_info(frame_info*, int, print_what, int, int) /home/simark/src/binutils-gdb/gdb/stack.c:914
    #9 0x55b6ef10e90d in print_stack_frame(frame_info*, int, print_what, int) /home/simark/src/binutils-gdb/gdb/stack.c:180
    #10 0x55b6ee9592f8 in print_stop_location /home/simark/src/binutils-gdb/gdb/infrun.c:7853
    #11 0x55b6ee95948f in print_stop_event(ui_out*) /home/simark/src/binutils-gdb/gdb/infrun.c:7870
    #12 0x55b6ef34b962 in tui_on_normal_stop /home/simark/src/binutils-gdb/gdb/tui/tui-interp.c:98
    #13 0x55b6ee01a14d in std::_Function_handler<void (bpstats*, int), void (*)(bpstats*, int)>::_M_invoke(std::_Any_data const&, bpstats*&&, int&&) /usr/include/c++/8.2.1/bits/std_function.h:297
    #14 0x55b6ee965415 in std::function<void (bpstats*, int)>::operator()(bpstats*, int) const /usr/include/c++/8.2.1/bits/std_function.h:687
    #15 0x55b6ee962f1b in gdb::observers::observable<bpstats*, int>::notify(bpstats*, int) const /home/simark/src/binutils-gdb/gdb/common/observable.h:106
    #16 0x55b6ee95a6e7 in normal_stop() /home/simark/src/binutils-gdb/gdb/infrun.c:8142
    #17 0x55b6ee93f236 in fetch_inferior_event(void*) /home/simark/src/binutils-gdb/gdb/infrun.c:3782
    #18 0x55b6ee8f2641 in inferior_event_handler(inferior_event_type, void*) /home/simark/src/binutils-gdb/gdb/inf-loop.c:43
    #19 0x55b6eea2a1f0 in handle_target_event /home/simark/src/binutils-gdb/gdb/linux-nat.c:4358
    #20 0x55b6ee7045f1 in handle_file_event /home/simark/src/binutils-gdb/gdb/event-loop.c:733
    #21 0x55b6ee704e89 in gdb_wait_for_event /home/simark/src/binutils-gdb/gdb/event-loop.c:859
    #22 0x55b6ee7027b5 in gdb_do_one_event() /home/simark/src/binutils-gdb/gdb/event-loop.c:322
    #23 0x55b6ee702907 in start_event_loop() /home/simark/src/binutils-gdb/gdb/event-loop.c:371
    #24 0x55b6eeadfc16 in captured_command_loop /home/simark/src/binutils-gdb/gdb/main.c:331
    #25 0x55b6eeae2ef9 in captured_main /home/simark/src/binutils-gdb/gdb/main.c:1174
    #26 0x55b6eeae30c2 in gdb_main(captured_main_args*) /home/simark/src/binutils-gdb/gdb/main.c:1190
    #27 0x55b6edf4fa89 in main /home/simark/src/binutils-gdb/gdb/gdb.c:32
    #28 0x7fed88ad8222 in __libc_start_main (/usr/lib/libc.so.6+0x24222)

gdb/ChangeLog:

	* source-cache.c (source_cache::get_source_lines): Re-read
	fullname after calling open_source_file.