Identifying bottom-of-stack

[Andrew Cagney]

>>>This would make debugging in the outermost frame quite annoying,
>>>> >wouldn't it?  We won't be able to find any frame relative variables.
>>
>>> 
>>> Why?  Frame relative variables us DW_AT_frame_base.
> 
> 
> You're right.  I still have the nagging feeling there's some other
> reason it matters, but since I can't think of specifics I'm probably
> just hallucinating again.

Two things that might help:

- dwarf2-frame incorrectly returns the CFA as the ``frame_base''.  Other 
stuff means that the value isn't used (well except for info frame).

- this relies on GDB permitting an unwind into, but not past, a frame 
with a NULL ID.  The old code would have stopped things one frame short.

Andrew