virtio-gpu: avoid possible NULL pointer dereference

If output is NULL it is not permissable to dereference it.
So we should leave the respective function in this case.

The inconsistency was indicated by cppcheck.
No actual NULL pointer dereference was observed.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Message-id: 1471813566-5324-1-git-send-email-xypron.glpk@gmx.de
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_plane.c

@@ -76,7 +76,8 @@ static void virtio_gpu_primary_plane_update(struct drm_plane *plane,
 		output = drm_crtc_to_virtio_gpu_output(plane->state->crtc);
 	if (old_state->crtc)
 		output = drm_crtc_to_virtio_gpu_output(old_state->crtc);
-	WARN_ON(!output);
+	if (WARN_ON(!output))
+		return;
 
 	if (plane->state->fb) {
 		vgfb = to_virtio_gpu_framebuffer(plane->state->fb);
@@ -129,7 +130,8 @@ static void virtio_gpu_cursor_plane_update(struct drm_plane *plane,
 		output = drm_crtc_to_virtio_gpu_output(plane->state->crtc);
 	if (old_state->crtc)
 		output = drm_crtc_to_virtio_gpu_output(old_state->crtc);
-	WARN_ON(!output);
+	if (WARN_ON(!output))
+		return;
 
 	if (plane->state->fb) {
 		vgfb = to_virtio_gpu_framebuffer(plane->state->fb);