compositor: Silence compiler warning by initializing resource to NULL

Although resource will be valid when it is used since the entered and left
masks are subsets of the different mask it would be nice not to have a
spurious compiler warning.

src/compositor.c

@@ -1191,7 +1191,7 @@ weston_surface_update_output_mask(struct weston_surface *es, uint32_t mask)
 	uint32_t entered = mask & different;
 	uint32_t left = es->output_mask & different;
 	struct weston_output *output;
-	struct wl_resource *resource;
+	struct wl_resource *resource = NULL;
 	struct wl_client *client = es->surface.resource.client;
 
 	if (es->surface.resource.client == NULL)