Avoid segfault for selectable tab labels (even without a segfault, they

Avoid segfault for selectable tab labels (even without a segfault, they

	* gtk/gtknotebook.c (gtk_notebook_set_focus_child): Avoid segfault
	for selectable tab labels (even without a segfault, they make the
	notebook almost unusable, though).  (#69985)

	* gtk/gtkmenuitem.c (gtk_menu_item_set_accel_path): Silently do
	nothing if the parent is not a menu.  (#66492)

	* gtk/gtkitemfactory.c (gtk_item_factory_from_widget,
	gtk_item_factory_path_from_widget): Try fetching the return value
	from menu_item->submenu as a fallback.  (#69020)