Do not segv if NULL is passed for CTX in sendfd and receivefd.

* src/assuan-buffer.c (assuan_sendfd): Check that CTX is not NULL.
(assuan_receivefd): Ditto.

This change is to align the behaviour with that of assuan_send_data.

• Suggested-by: Andre Heinecke.
• Signed-off-by: Werner Koch <wk@gnupg.org>