bug#68940: 29.2; Random crashes in face for char / font

[Eli Zaretskii]

> From: Robert Vojta <rvojta@me.com>
> Cc: 68940@debbugs.gnu.org
> Date: Tue, 06 Feb 2024 16:55:02 +0100
> 
> ... not counting the number of attempts, but it took me +1 hour to
> reproduce it. Here's the better backtrace:
> 
>   https://gist.github.com/zrzka/b01a3c8c013c5efcafd9fde6ac8b78a9
> 
> Now, I have two things to do:
> 
>  - Still figuring out the minimal init.el to reproduce
>  
>  - Build Emacs again, with all optimizations disabled, to be able to
>    step through frames and see what's going on

Yes, what's important is to see what is value of base_face at entry to
face_for_font -- it sounds like that's an invalid face doer some
reason.  Do you have any customizations of the fontsets?