Check for window being destroyed

	* src/sendmsg-window.c (sw_close_activated): mark window as
	being destroyed;
	(sw_get_action): check for window being destroyed, and return
	NULL if so;
	(sw_action_set_enabled), (sw_action_get_enabled),
	(sw_action_set_active), (sw_action_get_active): check for NULL
	action.