* sentinel-frame.h, sentinel-frame.c: New files.
* Makefile.in (frame.o): Update dependencies.
(SFILES): Add sentinel-frame.c.
(sentinel_frame_h): Define.
(COMMON_OBS): Add sentinel-frame.o.
(sentinel-frame.o): Specify dependencies.
* frame.c: Include "sentinel-frame.h".
(frame_register_unwind): Rewrite assuming that there is always a a
->next frame.
(frame_register, generic_unwind_get_saved_register): Ditto.
(frame_read_unsigned_register, frame_read_signed_register): Ditto.
(create_sentinel_frame, unwind_to_current_frame): New functions.
(get_current_frame): Rewrite using create_sentinel_frame and
unwind_to_current_frame. When possible, always create a frame.
(create_new_frame): Set next to the sentinel frame.
(get_next_frame): Rewrite. Don't go below the level 0 frame.
(deprecated_update_frame_pc_hack): Update the next frame's PC and
ID cache when necessary.
(frame_saved_regs_id_unwind): Use frame_relative_level.
(deprecated_generic_get_saved_register): Use frame_relative_level,
get_frame_saved_regs, get_frame_pc, get_frame_base and
get_next_frame.
(frame_saved_regs_register_unwind): Use get_frame_saved_regs and
frame_register.
