* thread.c (restore_selected_frame): Handle frame_level == -1.

	* thread.c (restore_selected_frame): Handle frame_level == -1.
	(make_cleanup_restore_current_thread): Use
	get_selected_frame_if_set.
	* frame.h (get_selected_frame_if_set): Declare.
	* frame.c (get_selected_frame_if_set): New function.