[gdb] Fix heap-buffer-overflow in find_program_interpreter

[gdb] Fix heap-buffer-overflow in find_program_interpreter

With the test-case included in this patch, we run into:
...
(gdb) target remote localhost:2347^M
`target:twice-connect' has disappeared; keeping its symbols.^M
Remote debugging using localhost:2347^M
warning: Unable to find dynamic linker breakpoint function.^M
GDB will be unable to debug shared library initializers^M
and track explicitly loaded dynamic code.^M
Reading /usr/lib/debug/.build-id/$hex/$hex.debug from remote target...^M
0x00007ffff7dd4550 in ?? ()^M
(gdb) PASS: gdb.server/twice-connect.exp: session=second: gdbserver started
FAIL: gdb.server/twice-connect.exp: found interpreter
...

The problem originates in find_program_interpreter, where
bfd_get_section_contents is called to read .interp, but fails.  The function
returns false but the result is ignored, so find_program_interpreter returns
some random string.

Fix this by checking the result of the call to bfd_get_section_contents.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29652