bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Eli Zaretskii]

> Cc: 41321@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sat, 30 May 2020 11:04:55 -0700
> 
> For emacs-27 I propose the attached, more-conservative patch instead. 
> This is a backport of part of a patch I've been working on for master. 
> As part of that effort I've found some other obscure GC-related bugs 
> that we've been lucky to avoid; this patch focuses only on the area Eli 
> encountered.

Please explain in comments why we are marking one more pointer in the
loop.  Also, I don't think I understand why this solves all of the
problems we were discussing; is this in addition to another patch that
you propose for emacs-27?

Thanks.