bug#28279: 25.2; repeatable crash when hitting C-g twice

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Cc: yuri@tenuki.org,  28279@debbugs.gnu.org
> Date: Thu, 31 Aug 2017 12:48:26 -0400
> 
> Eli Zaretskii wrote:
> 
> > Does the below look reasonable for a fix?
> 
> Sorry, I don't know. If you want a second opinion, ask Paul?

I just wrote code that does the same as the original one, but calls
assq_no_quit instead of Fassq and CDR_SAFE instead of Fcdr.  This
should not signal an error nor QUIT, which was the root cause for the
abort.