bug#38442: 27.0.50; segmentation fault switching to cairo

[Eli Zaretskii]

> Date: Sun, 01 Dec 2019 05:38:06 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 38442@debbugs.gnu.org
> 
> > From: Ola Nilsson <ola.nilsson@gmail.com>
> > Date: Sun, 1 Dec 2019 00:03:26 +0100
> > 
> > I recently rebuilt my Emacs from master and also enabled cairo with
> > --with-cairo.  Something in my desktop file triggered a segmentation
> > fault with the backtrace below
> 
> You switched to Cairo, but the desktop file mentions font back-ends
> not available with Cairo:
> 
> > (setq desktop-saved-frameset [frameset 1 (24034 55364 390522 65000) 
> > (desktop . "208") "olani@blue" nil nil
> > ((((minibuffer . t) (undecorated) (override-redirect) (font-backend xft x) 
> > (font . "-PfEd-DejaVu Sans
>   
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Juanma, I think given these problems we should filter out font-backend
when restoring a frameset.  If you agree, can you make this change,
please?

Thanks.