bug#28268: 26.0.50; (MS Windows) crash on C-g after closing Git GUI

[Eli Zaretskii <eliz@gnu.org>]

> From: Richard Copley <rcopley@gmail.com>
> Date: Mon, 28 Aug 2017 22:10:56 +0100
> 
> Emacs sometimes crashes when I type C-g just after closing a process.
> I've only seen this happen after the following recipe using Magit on MS
> Windows. (Not sure whether or not Magit is essential.)
> 
> Recipe:
> 
> Visit a git repo.
> C-x g ; magit-status
> ! g   ; magit-run-popup, magit-run-git-gui
> ;; A nasty-looking window pops up (Git GUI, I presume). Close it.
> C-g   ; Crashes.

I couldn't reproduce this here: the Git GUI didn't pop up for me.
Perhaps something is missing from the recipe ("C-x g" was also
unbound), or maybe it's because my Git is configured to be run only
from Git Bash, not from anywhere else on Windows.

However, I installed a change which might fix this problem.  Please
test the current master.  Also, just so I'm sure I didn't miss
anything, please post the backtrace from all the threads ("thread
apply all bt" at GDB prompt), from the binary where you get these
aborts.

Thanks.