crypto: algif_hash - Fix NULL hash crash with shash

Recently algif_hash has been changed to allow null hashes.  This
triggers a bug when used with an shash algorithm whereby it will
cause a crash during the digest operation.

This patch fixes it by avoiding the digest operation and instead
doing an init followed by a final which avoids the buggy code in
shash.

This patch also ensures that the result buffer is freed after an
error so that it is not returned as a genuine hash result on the
next recv call.

The shash/ahash wrapper code will be fixed later to handle this
case correctly.

Fixes: 493b2ed3f760 ("crypto: algif_hash - Handle NULL hashes correctly")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Laura Abbott <labbott@redhat.com>