bug#39133: 28.0.50; Emacs slowdown on special char

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: lg.zevlg@gmail.com,  39133@debbugs.gnu.org,
>   mituharu@math.s.chiba-u.ac.jp,  handa@gnu.org
> Date: Fri, 24 Jan 2020 14:09:22 +0100
> 
>     >> Rather than duplicate that code, I moved the FONT_INVALID_CODE check
>     >> up. This works for me:
> 
>     Eli> Looks reasonable, thanks.
> 
> Except it crashes under -nw.

Yes, because the call to the font driver should only be done when
'font_object' is a font object (it isn't on TTY frames).  Otherwise,
simply assign FONT_INVALID_CODE to 'code'.