Avoid burn_stack in CAST5 setkey

* cipher/cast5.c (do_cast_setkey): Use wipememory instead of memset.
(cast_setkey): Remove stack burning.

Burning stack does not work properly when compiler inlines static functions,
therefore use wipememory to clear stack after use instead of relying on
_gcry_burn_stack.

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