bug#5662: flet not undone on lisp nesting error

[Stefan Monnier <monnier@iro.umontreal.ca>]

> If I trigger a lisp nesting error with an infinite recursion inside a
> let and an flet binding, then the effects of the flet are not undone,
> resulting in a change of binding at the top-level.

> (defun g () 'g-orig)
> (setq a 'a-orig)

> (defun h ()
>   (let ((a 'a-new))
>     (flet ((g () 'g-new))
>       (h))))
> (h)   ;; <-- Lisp nesting exceeds `max-lisp-eval-depth'
> (g)   ;; g-new ! 
> a     ;; a-orig 

I can indeed reproduce it.  I'm not sure yet what's going on, but it
might be due to the "Lisp nesting exceeds `max-lisp-eval-depth'" error
happening in the unwind-protect code (i.e. while undoing the `g'
binding).  The explanation can't be quite so simple, but my gut feeling
tells me it's got to do with it.


        Stefan