bug#23386: Segfault when messing with font-backend

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 23386@debbugs.gnu.org
> From: Dmitry Antipov <dmantipov@yandex.ru>
> Date: Tue, 12 Jul 2016 18:20:41 +0300
> 
> > But it doesn't do what the user asked for.
> >
> > I don't see why it would be meaningless to evict a backend and start
> > using another one, if all it takes is re-open a bunch of fonts.
> 
> OK, the following patch basically works for me (not tested too much, BTW).

Thanks.

> --- a/src/frame.c
> +++ b/src/frame.c
> @@ -3712,7 +3712,11 @@ x_set_font_backend (struct frame *f, Lisp_Object new_value, Lisp_Object old_valu
>      return;
>  
>    if (FRAME_FONT (f))
> -    free_all_realized_faces (Qnil);
> +    {
> +      Lisp_Object frame;
> +      XSETFRAME (frame, f);
> +      free_all_realized_faces (frame);
> +    }

Since free_all_realized_faces with a nil argument will free faces on
all frames, can you tell why this hunk was needed?