bug#81568: 32.0.50; crash in Fnext_single_property_change

> Date: Thu, 06 Aug 2026 19:09:52 +0300
> From: Andrei Elkin via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <[email protected]>
> 
> I got a crash when my last activity was selecting a piece of text with
> mouse.
> Emacs process is still alive in gdb for inspection by who volunteers.
> The stacks - bt, xbacktrace, bt full - placed below.

Please try to figure out which data caused the segfault.  The code
where it happens is

  next = next_interval (i);
  while (next
	 && EQ (here_val, textget (next->plist, prop))
	 && (NILP (limit) || next->position < XFIXNUM (limit)))
    next = next_interval (next);

and according to the backtrace, the value of 'next' looks okay at
first sight.