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

[Robert Vojta]

Small progress ... Compiled same version from source. Running
"src/emacs" makes it really hard to manually reproduce it. The crash
rate is very very low. I've added ...

  (run-with-timer (random 5) 0 'kill-emacs)

... at the end of my init.el file, and then ...

  while true ; do lldb --batch -o run -f src/emacs ; sleep 1; done

... 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