bug#36431: Crash in marker.c:337

[Eli Zaretskii]

> Date: Sat, 29 Jun 2019 15:13:26 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 36431@debbugs.gnu.org
> 
> > Date: Sat, 29 Jun 2019 13:17:34 +0200 (CEST)
> > From: Werner LEMBERG <wl@gnu.org>
> > 
> > while sending an e-mail with `mew', I get the attached crash.  This is
> > something new, which I haven't experienced before.  I can reliably
> > repeat it.
> 
> Please send the reproduction recipe, preferably starting from "emacs
> -Q", including any files required for the reproduction.  I don't think
> it's possible to debug this otherwise, as the problem is clearly
> data-dependent (somehow we end up requesting a byte-to-character
> position conversion for a byte position that is not on character
> boundary).

CC'ing Stefan, as it turns out he added that assertion not long ago.