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.

-Alan