Do not crash when calling GDB with empty executable name.

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.