layersurface: Fix wrong assertion

We want to check for equality. This avoids a leak.

src/layersurface.c

@@ -71,7 +71,7 @@ static void layer_surface_closed (void                         *data,
   PhoshLayerSurface *self = data;
   PhoshLayerSurfacePrivate *priv = phosh_layer_surface_get_instance_private (self);
 
-  g_return_if_fail (priv->layer_surface != surface);
+  g_return_if_fail (priv->layer_surface == surface);
   zwlr_layer_surface_v1_destroy(priv->layer_surface);
   priv->layer_surface = NULL;
   gtk_widget_destroy (GTK_WIDGET (self));