crypto: sahara - fix ahash selftest failure

update() calls should not modify the result buffer, so add an additional
check for "rctx->last" to make sure that only the final hash value is
copied into the buffer.

Fixes the following selftest failure:
alg: ahash: sahara-sha256 update() used result buffer on test vector 3,
cfg="init+update+final aligned buffer"

Fixes: 5a2bb93f ("crypto: sahara - add support for SHA1/256")
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/sahara.c

@@ -1047,7 +1047,7 @@ static int sahara_sha_process(struct ahash_request *req)
 
 	memcpy(rctx->context, dev->context_base, rctx->context_size);
 
-	if (req->result)
+	if (req->result && rctx->last)
 		memcpy(req->result, rctx->context, rctx->digest_size);
 
 	return 0;