crypto: shash - Fix zero-length shash ahash digest crash

The shash ahash digest adaptor function may crash if given a
zero-length input together with a null SG list.  This is because
it tries to read the SG list before looking at the length.

This patch fixes it by checking the length first.

Cc: <stable@vger.kernel.org>
Reported-by: Stephan Müller<smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Stephan Müller <smueller@chronox.de>

crypto/shash.c

@@ -275,12 +275,14 @@ static int shash_async_finup(struct ahash_request *req)
 
 int shash_ahash_digest(struct ahash_request *req, struct shash_desc *desc)
 {
-	struct scatterlist *sg = req->src;
-	unsigned int offset = sg->offset;
 	unsigned int nbytes = req->nbytes;
+	struct scatterlist *sg;
+	unsigned int offset;
 	int err;
 
-	if (nbytes < min(sg->length, ((unsigned int)(PAGE_SIZE)) - offset)) {
+	if (nbytes &&
+	    (sg = req->src, offset = sg->offset,
+	     nbytes < min(sg->length, ((unsigned int)(PAGE_SIZE)) - offset))) {
 		void *data;
 
 		data = kmap_atomic(sg_page(sg));