Better fix for perl #107440

Better fix for perl #107440

> > Actually, the simplest solution seem to be to put the av or hv on
> > the mortals stack in pp_aassign and pp_undef, rather than in
> > [ah]v_undef/clear.
>
> This makes me nervous. The tmps stack is typically cleared only on
> statement boundaries, so we run the risks of
>
>     * user-visible delaying of freeing elements;
>     * large tmps stack growth might be possible with
>       certain types of loop that repeatedly assign to an array without
>       freeing tmps (eg map? I think I fixed most map/grep tmps leakage
> a
>       while back, but there may still be some edge cases).
>
> Surely an ENTER/SAVEFREESV/LEAVE inside pp_aassign is just as
> efficient,
> without any attendant risks?
>
> Also, although pp_aassign and pp_undef are now fixed, the
> [ah]v_undef/clear functions aren't, and they're part of the public API
> that can be called independently of pp_aassign etc. Ideally they
> should
> be fixed (so they don't crash in mid-loop), and their documentation
> updated to point out that on return, their AV/HV arg may have been
> freed.

This commit takes care of the first part; it changes pp_aassign to use
ENTER/SAVEFREESV/LEAVE and adds the same to h_freeentries (called both
by hv_undef and hv_clear), av_undef and av_clear.

It effectively reverts the C code part of 9f71cfe6ef2.