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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 30 Jul 2021 23:02:37 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 49782@debbugs.gnu.org
> 
> > 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.

But before you embark on that adventure, please try another stab in
the dark:

diff --git a/src/fontset.c b/src/fontset.c
index d2d428d..7d4bd65 100644
--- a/src/fontset.c
+++ b/src/fontset.c
@@ -1356,15 +1356,16 @@ check_fontset_name (Lisp_Object name, Lisp_Object *frame)
   int id;
   struct frame *f = decode_live_frame (*frame);
 
-  if (!FRAME_WINDOW_P (f) && !FRAME_INITIAL_P (f))
-    error ("Can't use fontsets in non-GUI frames");
-
   XSETFRAME (*frame, f);
 
   if (EQ (name, Qt))
     return Vdefault_fontset;
   if (NILP (name))
-    id = FRAME_FONTSET (f);
+    {
+      if (!FRAME_WINDOW_P (f))
+	error ("Can't use fontsets in non-GUI frames");
+      id = FRAME_FONTSET (f);
+    }
   else
     {
       CHECK_STRING (name);