tests:gpgscm: Raise an error correctly for process spawning.

* tests/gpgscm/ffi.c (do_process_spawn_io): Handle ERR.
(do_process_spawn_fd): Likewise.

• Reported-by: Marcel Telka <marcel@telka.sk>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>