bug#65796: dynamic module non_local_exit_get overwrites exit signals

[Eli Zaretskii]

> From: Xinyang Chen <chenxinyang99@gmail.com>
> Date: Wed, 6 Sep 2023 18:52:14 -0400
> 
> Currently `module_non_local_exit_get` returns pointers to fields
> in emacs_env_private:
> ```
>   if (p->pending_non_local_exit != emacs_funcall_exit_return)
>     {
>       *symbol = &p->non_local_exit_symbol;
>       *data = &p->non_local_exit_data;
>     }
> ```
> this means that if one tries to:
> ```
> funcall(...);
> non_local_exit_get(&s1, &d1);
> funcall(...);
> non_local_exit_get(&s2, &d2);
> non_local_exit_signal(s1, d1);
> ```
> you would signal the second error, instead of the first error (I expected
> this to happen).
> It seems to me that `module_non_local_exit_get` should
> `allocate_emacs_value` instead.

Philipp, Daniel: any comments?

Btw, the non_local_exit_get function is currently not documented in
the ELisp manual; should it be?