get_prev_frame, UNWIND_NULL_ID -> UNWIND_OUTERMOST

get_prev_frame, UNWIND_NULL_ID -> UNWIND_OUTERMOST

- The UNWIND_NULL_ID check in get_prev_frame_1 used to really be
  against null_frame_id, back before we had outer_frame_id.  We didn't
  have UNWIND_OUTERMOST when outer_frame_id was added, but we do now,
  and it's more accurate.

- It used to be necessary to check for the sentinel frame explicitly
  because that uses null_frame_id for frame id.  Since no other frame
  can have that id nowadays (it's asserted by compute_frame_id), we
  don't need that explicit check.

Tested on x86_64 Fedora 17.

gdb/
2013-11-28  Pedro Alves  <palves@redhat.com>

	* frame.c (get_prev_frame_1): If the frame id is outer_frame_id,
	set the unwind stop reason to UNWIND_OUTERMOST, not
	UNWIND_NULL_ID.  Remove explicit check for sentinel frame.