crypto: akcipher - Do not copy dst if it is NULL

As signature verification has a NULL destination buffer, the pointer
needs to be checked before the memcpy is done.

Fixes: addde1f ("crypto: akcipher - Add sync interface without SG lists")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Author: herbertx

crypto/akcipher.c

@@ -234,7 +234,8 @@ EXPORT_SYMBOL_GPL(crypto_akcipher_sync_prep);
 int crypto_akcipher_sync_post(struct crypto_akcipher_sync_data *data, int err)
 {
 	err = crypto_wait_req(err, &data->cwait);
-	memcpy(data->dst, data->buf, data->dlen);
+	if (data->dst)
+		memcpy(data->dst, data->buf, data->dlen);
 	data->dlen = data->req->dst_len;
 	kfree_sensitive(data->req);
 	return err;