Fix 4K leak in open_source_file each time next/step changes of function.

Fix 4K leak in open_source_file each time next/step changes of function.

When current function changes after a next/step, GDB shows a message such as:
  (gdb) s
  info_fun1 ()
      at /bd/home/philippe/gdb/git/build_smallthing/gdb/testsuite/../../../smallthing/gdb/testsuite/gdb.base/info_qt.c:41
  41	  info_qt_inc++;
  (gdb)

Valgrind reports a 4K definite leak for each such message (full stacktrace of
the leak below).

This patch fixes this leak, by transferring the current s->fullname to the
unique_xmalloc_ptr fullname given to find_and_open_source.

Note that I do not understand why find_and_open_source always tries to
re-execute the substitution rules on the provided fullname, as source.c
symtab_to_fullname just blindly returns a non NULL s->fullname, counting on
forget_cached_source_info to be called if search dir or substitution rules are
changed.  Similarly, psymtab_to_fullname also just returns a non NULL
ps->fullname.

==15309== VALGRIND_GDB_ERROR_BEGIN
==15309== 69,632 bytes in 17 blocks are definitely lost in loss record 3,158 of 3,186
==15309==    at 0x4C2BE2D: malloc (vg_replace_malloc.c:299)
==15309==    by 0x5BF0987: realpath@@GLIBC_2.3 (canonicalize.c:78)
==15309==    by 0x41F713: gdb_realpath(char const*) (pathstuff.c:72)
==15309==    by 0x608833: openp(char const*, enum_flags<openp_flag>, char const*, int, std::unique_ptr<char, gdb::xfree_deleter<char> >*) (source.c:861)
==15309==    by 0x608B89: find_and_open_source(char const*, char const*, std::unique_ptr<char, gdb::xfree_deleter<char> >*) (source.c:1049)
==15309==    by 0x608D0B: open_source_file(symtab*) (source.c:1074)
==15309==    by 0x609101: print_source_lines_base(symtab*, int, int, enum_flags<print_source_lines_flag>) (source.c:1291)
==15309==    by 0x614ADF: print_frame_info(frame_info*, int, print_what, int, int) (stack.c:911)
==15309==    by 0x614C45: print_stack_frame(frame_info*, int, print_what, int) (stack.c:181)
==15309==    by 0x511D5E: print_stop_location (infrun.c:8044)
==15309==    by 0x511D5E: print_stop_event(ui_out*) (infrun.c:8061)
==15309==    by 0x40DD6D: cli_on_normal_stop(bpstats*, int) (cli-interp.c:145)
==15309==    by 0x512409: operator() (functional:2127)
==15309==    by 0x512409: notify (observable.h:106)
==15309==    by 0x512409: normal_stop() (infrun.c:8334)
==15309==    by 0x5156D8: fetch_inferior_event(void*) (infrun.c:3955)
==15309==    by 0x4B3EEC: gdb_wait_for_event(int) (event-loop.c:859)
==15309==    by 0x4B3FF6: gdb_do_one_event() [clone .part.4] (event-loop.c:322)
==15309==    by 0x4B41B4: gdb_do_one_event (common-exceptions.h:219)
==15309==    by 0x4B41B4: start_event_loop() (event-loop.c:371)
==15309==    by 0x551217: captured_command_loop() (main.c:330)
==15309==    by 0x55220C: captured_main (main.c:1177)
==15309==    by 0x55220C: gdb_main(captured_main_args*) (main.c:1193)
==15309==    by 0x29B4F7: main (gdb.c:32)
==15309==
==15309== VALGRIND_GDB_ERROR_END

gdb/ChangeLog
2018-11-04  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* source.c (open_source_file): Fix leak by transferring the
	current s->fullname to the unique_xmalloc_ptr fullname given
	to find_and_open_source.