bug#26932: Found the triggering behaviour

[Vivek Dasmohapatra]

> If I understand correctly, the external lui package set buffer-undo-list
> to something invalid, and this led Emacs to segfault when gc-ing?

Not invalid as such - entries were removed from the list while it was "in 
flight" through a mapc/mapcar and a double free (or similar) occurred.

> Was there any further work done here, or a reproducible test case
> written?

No test case was written, the code for lui was changed to operate on
a copy of the undo list, then assign that back to the original head 
variable instead.