bug#31245: 27.0.50; Crash in Windows emacs

[Eli Zaretskii <eliz@gnu.org>]

> From: Andy Moreton <andrewjmoreton@gmail.com>
> Date: Tue, 01 May 2018 21:47:00 +0100
> 
> On Mon 23 Apr 2018, Andy Moreton wrote:
> 
> > On Mon 23 Apr 2018, Eli Zaretskii wrote:
> >> Thanks, but this kind of problems is impossible to debug without a
> >> reproducer.  Can you come up with one?
> 
> I still cannot use a debugger on this system, but I have collected two
> more backtraces from emacs built with the same options as before, from
> changeset 65ac27783a959a8339c2aab0f1e54d9b508a1f1f on master.
> 
> In both of these I was typing in an rnc-mode buffer (for a Relax-NG
> schema), and it crashed while I typed OSVERSION as an element name in
> the schema (only the O was displayed). The file is plain ascii.

Thanks.  There's no need to run Emacs under a debugger in this case,
AFAIU.  What is needed is a recipe that would more or less reliably
reproduce the problem.  The place where the assertion is violated and
why Emacs aborts is quite clear; what is missing is the root cause of
that impossible situation.  And that can only be provided by a recipe.

Can you describe what should one do to reproduce this problem,
starting from "emacs -Q", loading rnc-mode and anything else that is
needed, then typing or evaluating something that causes the crash?