bug#21666: Bug is indeed undo-list and GC related

[Eli Zaretskii]

> From: Jorgen Schäfer <jorgen.schaefer@gmail.com>
> Date: Wed, 09 Mar 2016 10:14:35 +0000
> 
> I do not know how GC works in Emacs Lisp, but it seems to me that an Emacs 
> Lisp program should not be
> able to segfault Emacs in this manner. It is interesting that this problem 
> was introduced sometime after
> 19efb9 – an Emacs built from that commit did not exhibit the problem.

Can you bisect it more narrowly?  "After 19efb9" leaves a lot of
alternatives...

Thanks.