[rfc] Add assert to catch get_frame_arch (NULL) calls

[Joel Brobecker]

Hmpf, should have read the rest of my gdb-patches mail before answering
your previous commit - I'm just a little tied up right now, so I just
browse some emails semi-randomly when I have a few minutes of free time.

> ChangeLog:
> 
> 	* frame.c (get_frame_arch): Abort if called with NULL this_frame.

This would fine for me. I'm slightly concerned about the next release,
whether this might introduce an unnecessary risk or not.  But this
will depend on the amount of time that we will take before creating
the next branch.  I will add that to the list of things to reassess
before we make the next release.

-- 
Joel