md: clear context with wipememory to avoid false warning

* cipher/md.c (md_open): Initialize ctx with wipememory2 instead
of memset.

GCC 11 gives warning on use of memset:
warning: '__builtin_memset' offset [0, 19] is out of the bounds [0, 0] [-Warray-bounds]

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>