bug#23386: Segfault when messing with font-backend

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Date: Wed, 27 Apr 2016 09:13:30 -0400
> 
> If I do
> 
>     % emacs -Q --eval "(push '(font-backend x) default-frame-alist)"
> 
> I get a segfault.  This is with the latest emacs-25.

Not reproducible here, but then I cannot run an X version.  (I did try
an equivalent for Windows; it didn't crash.)

> See the backtrace below.
> 
> 
>         Stefan
> 
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0xf70b3b36 in XftCharIndex () from /usr/lib/i386-linux-gnu/libXft.so.2
> (gdb) xbacktrace
> (gdb) bt
> #0  0xf70b3b36 in XftCharIndex () from /usr/lib/i386-linux-gnu/libXft.so.2
> #1  0x082d4d69 in xftfont_encode_char (font=0x8779b00, c=92) at xftfont.c:537

Can you tell which arguments to XftCharIndex were invalid in this
case?

Also, is 'x' a valid font-backend symbol?  It's strange that Emacs
uses xftfont when it should have been using xfont instead (AFAIU).
But I'm far from being an expert in this area.

Thanks.