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

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Sun, 17 May 2020 10:56:28 +0000
> Cc: 41321@debbugs.gnu.org
> 
> What I would do next is run with a breakpoint on wrong_type_argument
> (if that's impossible, change the code in CHECK_MARKER to abort upon
> encountering a PVEC_FREE vector) to see where the reference to the
> freed pseudovector came from. An undo list, maybe?

I'm already running with such a breakpoint, let's how it will catch
something.