crypto: hash - Zap unaligned buffers

Some unaligned buffers on the stack weren't zapped properly which
may cause secret data to be leaked.  This patch fixes them by doing
a zero memset.

It is also possible for us to place random kernel stack contents
in the digest buffer if a digest operation fails.  This is fixed
by only copying if the operation succeeded.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>