xen_disk: fix possible null-ptr dereference

Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/block/xen_disk.c

@@ -589,6 +589,7 @@ static int blk_send_response_one(struct ioreq *ioreq)
         break;
     default:
         dst = NULL;
+        return 0;
     }
     memcpy(dst, &resp, sizeof(resp));
     blkdev->rings.common.rsp_prod_pvt++;