bug#26650: 26.0.50; Protect *Backtrace* from accidental killing

[Eli Zaretskii]

> From: Tino Calancha <tino.calancha@gmail.com>
> Date: Wed, 26 Apr 2017 19:36:54 +0900 (JST)
> cc: npostavs@users.sourceforge.net, Tino Calancha <tino.calancha@gmail.com>, 
>     26650@debbugs.gnu.org
> 
> >> +  (add-hook 'kill-buffer-hook
> >> +            (lambda () (if (> (recursion-depth) 0) (top-level))) nil t)
> >
> > This will throw to top-level when _any_ buffer is killed, as long as
> > we are in recursive-edit, no?
> No, because `add-hook' is called with non-nil LOCAL arg.

Right, sorry for not paying attention.