bug#36166: 27.0.50; Crash visiting HELLO when fringe-mode is disabled

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: 36166@debbugs.gnu.org
> Date: Tue, 11 Jun 2019 16:51:31 +0200
> 
> w32 and X both check s->cmp, at least (although even then they check
> slightly different conditions), and they also handle COMPOSITE_GLYPH
> differently.
> 
> Itʼs easy enough to chuck in a check for font being NULL, but that
> feels like the wrong solution (and I donʼt know this code at all).

I think the important part is the test of s->font_not_found_p.  Can
you see whether it's zero in the crash case?