bug#23386: Segfault when messing with font-backend

[Eli Zaretskii]

> From: npostavs@users.sourceforge.net
> Cc: 23386@debbugs.gnu.org,  Eli Zaretskii <eliz@gnu.org>
> Date: Sat, 09 Jul 2016 16:02:30 -0400
> 
> > Here is a patch that attempts to fix the issue by resetting the font
> > after the backend is changed.  It does let Emacs successfully open the
> > frame with the new font (no funny box characters in the modeline), but
> > I'm not sure if it's the best way of marking the font object invalid.
> 
> Definitely not the best way: it causes segfault on delete-frame.

Backtrace from that segfault?

I think one idea that could be useful is to trace the creation of
relevant objects when Emacs starts up, starting with the call to
font_update_drivers, and then compare that with what happens in this
case.  That could delineate the missing parts and the differences
which could point the way to solving this cleanly.

Thanks.