bug#31692: Emacs sometimes drops key events

[Eli Zaretskii]

> From: Michael Heerdegen <michael_heerdegen@web.de>
> Cc: Eli Zaretskii <eliz@gnu.org>,  radon.neon@gmail.com,  
> 31692@debbugs.gnu.org
> Date: Sat, 09 Jun 2018 00:17:47 +0200
> 
> I tried to adapt Eli's patch accordingly.  To avoid confusion, I simply
> use the concrete value - the CATCH-SYM

I actually prefer my original code in this case, because it's closer
to what process_quit_flag does when quit-flag is tested while BODY
runs.

Thanks.