MenuButton: popover connects to menu_deactivate_cb

This was missed (even before my recent patches).

https://gitlab.gnome.org/GNOME/gtk/issues/199

gtk/gtkmenubutton.c

@@ -1049,6 +1049,9 @@ gtk_menu_button_dispose (GObject *object)
 
   if (priv->popover)
     {
+      g_signal_handlers_disconnect_by_func (priv->popover,
+                                            menu_deactivate_cb,
+                                            object);
       g_signal_handlers_disconnect_by_func (priv->popover,
                                             popover_destroy_cb,
                                             object);