virtio-gpu: fix memory leak in resource attach backing

In the resource attach backing function, everytime it will
allocate 'res->iov' thus can leading a memory leak. This
patch avoid this.
Signed-off-by: NLi Qiang <liq3ea@gmail.com>
Message-id: 1483003721-65360-1-git-send-email-liq3ea@gmail.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/display/virtio-gpu.c

@@ -714,6 +714,11 @@ virtio_gpu_resource_attach_backing(VirtIOGPU *g,
         return;
     }
 
+    if (res->iov) {
+        cmd->error = VIRTIO_GPU_RESP_ERR_UNSPEC;
+        return;
+    }
+
     ret = virtio_gpu_create_mapping_iov(&ab, cmd, &res->addrs, &res->iov);
     if (ret != 0) {
         cmd->error = VIRTIO_GPU_RESP_ERR_UNSPEC;