Don't access NULL by wipememory.

* src/assuan-inquire.c (assuan_inquire): Check mb->buf.

There is a case where mb->buf may be cleared already by get_membuf.

• Fixes-commit: d812e28af637144d078005887cb7519d7a2d0c58
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>