Guard against 'current_directory == NULL' on gdb_abspath (PR gdb/23613)

Guard against 'current_directory == NULL' on gdb_abspath (PR gdb/23613)

Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1728147
Ref.: https://sourceware.org/bugzilla/show_bug.cgi?id=23613

Hi,

This bug has been reported against Fedora GDB, but there's also an
upstream bug.  The problem reported is that GDB segfaults when the
working directory is deleted.  It's pretty use to reproduce it:

  mkdir bla
  cd bla
  rmdir ../bla
  gdb echo

Debugging the problem is a bit tricky, because, since the current
directory doesn't exist anymore, a corefile cannot be saved there.
After a few attempts, I came up with the following:

  gdb -ex 'shell mkdir bla' -ex 'cd bla' -ex 'shell rmdir ../bla' -ex 'r echo' ./gdb/gdb

This assumes that you're inside a build directory which contains
./gdb/gdb, of course.

After investigating it, I found that the problem happens at
gdb_abspath, where we're dereferencing 'current_directory' without
checking if it's NULL:

    ...
    (concat (current_directory,
	     IS_DIR_SEPARATOR (current_directory[strlen (current_directory) - 1])
	     ? "" : SLASH_STRING,
    ...

So I fixed the problem with the patch below.  The idea is that, if
'current_directory' is NULL, then the final string returned should be
just the "path".

After fixing the bug, I found a similar one reported against our
bugzilla: PR gdb/23613.  The problem is the same, but the reproducer
is a bit different.

I really tried writing a testcase for this, but unfortunately it's
apparently not possible to start GDB inside a non-existent directory
with DejaGNU.

I regression tested this patch on the BuildBot, and no regressions
were found.

gdb/ChangeLog:
2019-12-14  Sergio Durigan Junior  <sergiodj@redhat.com>

	https://bugzilla.redhat.com/show_bug.cgi?id=1728147
	PR gdb/23613
	* bsd-kvm.c (bsd_kvm_target_open): Use 'gdb_abspath'.
	* corelow.c: Include 'gdbsupport/pathstuff.h'.
	(core_target_open): Use 'gdb_abspath'.
	* gdbsupport/pathstuff.c (gdb_abspath): Guard against
	'current_directory == NULL' case.
	* gdbsupport/pathstuff.h (gdb_abspath): Expand comment and
	explain what happens when 'current_directory' is NULL.
	* go32-nat.c (go32_nat_target::wait): Check if
	'current_directory' is NULL before call to 'chdir'.
	* source.c (add_path): Use 'gdb_abspath'.
	* top.c: Include 'gdbsupport/pathstuff.h'.
	(init_history): Use 'gdb_abspath'.
	(set_history_filename): Likewise.
	* tracefile-tfile.c: Include 'gdbsupport/pathstuff.h'.
	(tfile_target_open): Use 'gdb_abspath'.

Change-Id: Ibb0932fa25bc5c2d3ae4a7f64bd7f32885ca403b