crypto: shash - Fix async finup handling of null digest

When shash_ahash_finup encounters a null request, we end up not
calling the underlying final function.  This patch fixes that.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/shash.c

@@ -240,12 +240,17 @@ int shash_ahash_finup(struct ahash_request *req, struct shash_desc *desc)
 	struct crypto_hash_walk walk;
 	int nbytes;
 
-	for (nbytes = crypto_hash_walk_first(req, &walk); nbytes > 0;
-	     nbytes = crypto_hash_walk_done(&walk, nbytes))
+	nbytes = crypto_hash_walk_first(req, &walk);
+	if (!nbytes)
+		return crypto_shash_final(desc, req->result);
+
+	do {
 		nbytes = crypto_hash_walk_last(&walk) ?
 			 crypto_shash_finup(desc, walk.data, nbytes,
 					    req->result) :
 			 crypto_shash_update(desc, walk.data, nbytes);
+		nbytes = crypto_hash_walk_done(&walk, nbytes);
+	} while (nbytes > 0);
 
 	return nbytes;
 }