Do not crash when calling GDB with empty executable name.

        % gdb -q ''

... or ...

        % gdb -q
        (gdb) file ''

... both cause GDB to crash with an invalid free. This is because
exec_file_attach is attempting to free a string that has not been
allocated. The string is only allocated if openp is successful.
But in the case of this obviously invalid filename, openp fails,
and leaves scratch_pathname uninitialized, thus causing the xfree
to fail.

The fix is to enable the associated cleanup after we have verified
that openp was successful.

gdb/ChangeLog (By Keith Seitz  <keiths@redhat.com>):

        * exec.c (exec_file_attach): Move cleanup after verifying that
        memory has in fact been allocated.

gdb/testsuite/ChangeLog:

        * gdb.base/empty_exe.exp: New testcase.

1 files changed, 2 insertions, 2 deletions

diff --git a/gdb/exec.c b/gdb/exec.c
index 615d5c0..21d3c3c 100644
--- a/gdb/exec.c
+++ b/gdb/exec.c
@@ -225,11 +225,11 @@ exec_file_attach (char *filename, int from_tty)
 	     &scratch_pathname);
 	}
 #endif
+      if (scratch_chan < 0)
+	perror_with_name (filename);
 
       cleanups = make_cleanup (xfree, scratch_pathname);
 
-      if (scratch_chan < 0)
-	perror_with_name (filename);
       if (write_files)
 	exec_bfd = gdb_bfd_fopen (scratch_pathname, gnutarget,
 				  FOPEN_RUB, scratch_chan);