Wipe the context before releasing as an extra safeguard.

* src/assuan-defs.h (wipememory2, wipememory): New.  Taken from GnuPG.
* src/assuan.c (assuan_release): Wipe the context.

The assuan context has buffers which may carry senitive information.
These buffers could be wiped out with each flush but that is too
expensive. Thus we only wipe them when freeing the context.

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