wayland/dma-buf: Handle getting dma-buf from detached buffer handle

We might still have a MetaWaylandBuffer for a wl_buffer that was
destroyed. Handle trying to fetch the MetaWaylandDmaBufBuffer from such
a buffer gracefully.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/798

src/wayland/meta-wayland-dma-buf.c

@@ -282,6 +282,9 @@ static const struct wl_buffer_interface dma_buf_buffer_impl =
 MetaWaylandDmaBufBuffer *
 meta_wayland_dma_buf_from_buffer (MetaWaylandBuffer *buffer)
 {
+  if (!buffer->resource)
+    return NULL;
+
   if (wl_resource_instance_of (buffer->resource, &wl_buffer_interface,
                                &dma_buf_buffer_impl))
     return wl_resource_get_user_data (buffer->resource);