bug#30931: 27.0.50; Crash in "Automatic GC"

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Fri, 30 Mar 2018 01:39:30 -0400
> Cc: 30931 <30931@debbugs.gnu.org>
> 
> So the following is enough to trigger the bug:
> 
>   (with-temp-buffer
>     (insert "1234567890")
>     (setq buffer-undo-list nil)
>     (narrow-to-region 2 5)
>     (save-restriction
>       (widen)
>       (delete-region 1 6))
>     (princ (format "%S" buffer-undo-list) #'external-debugging-output)
>     (type-of (car (nth 1 buffer-undo-list))))

Since Paul fixed the bug now (thanks, Paul!), would you like to add
this test to the test suite?

Thanks.