[patch/rfc] Add a sentinel frame

[Andrew Cagney]

>> When you first committed that stuff, I warned you that would happen :-)
>> The above test handled differently.
> 
> 
> Hey, you can't blame me for this bit.  I didn't add that check for
> DEPRECATED_PC_IN_CALL_DUMMY, it was already there in
> generic_frame_chain_valid.

I'm refering to frame_chain_valid(), a small part of which you changed. 
  The useful bits (your changes) were copied to the rewritten 
get_prev_frame.  When frame_chain_valid() is deleted, that duplication 
will go away.  To see what's wrong with frame_chain_valid() see 
legacy_get_prev_frame.

Andrew