bug#49782: 28.0.50; set-fontset-font crashes in init (server)

[Eli Zaretskii]

> From: Benjamin Riefenstahl <b.riefenstahl@turtle-trading.net>
> Cc: 49782@debbugs.gnu.org
> Date: Fri, 30 Jul 2021 21:26:12 +0200
> 
> Eli Zaretskii writes:
> > Sorry, should be fixed now.
> 
> But that additional condition re-introduces the original problem.

Then I'll need a backtrace from a live Emacs process with "xbacktrace"
results as well, and in a build without optimizations, so I could see
what's the difference between these two use cases.  (I cannot produce
such a backtrace myself, because your recipe involves 3 different
processes, one of which forks, which is hard to follow with GDB on a
system I have here.)