gbm: Reject buffers that are not wl_drm buffers in gbm_bo_import()

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>

src/gbm/backends/dri/gbm_dri.c

@@ -375,6 +375,9 @@ gbm_dri_bo_import(struct gbm_device *gbm,
    {
       struct wl_drm_buffer *wb = (struct wl_drm_buffer *) buffer;
 
+      if (!wayland_buffer_is_drm(buffer))
+         return NULL;
+
       image = wb->driver_buffer;
 
       switch (wb->format) {