bug#33014: 26.1.50; 27.0.50; Fatal error after re-evaluating a thread's function

[Eli Zaretskii]

> From: Gemini Lasswell <gazally@runbox.com>
> Cc: 33014@debbugs.gnu.org
> Date: Mon, 29 Oct 2018 11:24:10 -0700
> 
> Pushed to master, along with a new test which should fail if some future
> optimizing compiler removes the reference from the stack in spite of the
> 'volatile'.

Thanks.

Any reason not to cherry-pick this to emacs-26?