Fix a crash when stepping and unwinding fails

[Joel Brobecker]

> Of course, I was mostly hoping for a response from Mark Kettenis, and I
> believe he went on vacation a couple days later.  But I'd really like
> to have this issue fixed for GDB 6.5.  Does anyone object to my
> checking it in - and then, if Mark would like me to change it later,
> I'll be sure to take care of it promptly.

I was actually going to ask about this. No objection from me.

On a related note, there is also a change that you posted that
also fixed a crash for MinGW-hosted debugger, right? Stupidly,
I lost that message. Anyway, that might also be another interesting
fix for the release.

-- 
Joel