bug#33034: `unwind-protect' cleanup form is not executed if body dies in stack overflow

[Eli Zaretskii]

> From: Paul Pogonyshev <pogonyshev@gmail.com>
> Date: Sat, 13 Oct 2018 12:07:48 +0200
> 
> To reproduce:
> 
>     (defun overflow ()
>       (overflow))
>     (defun test ()
>       (interactive)
>       (message "BEFORE")
>       (unwind-protect
>           (overflow)
>         (message "CLEANUP")))
> 
> Invocation of `test' never issues message "CLEANUP", whether it is run
> interactively or non-interactively.

You are not supposed to continue using Emacs as usual after it
recovered from a C stack overflow.  You are supposed to exit Emacs and
restart the session.

The C stack overflow recovery is provided to allow you to save your
edits instead of losing them.

P.S.  I was somehow certain that we say the above somewhere in the
docs, but I cannot find it, so maybe I was dreaming.  Patches to add
that are welcome.