[perl #106282] Don’t crash cloning tied %^H

[perl #106282] Don’t crash cloning tied %^H

When hv_iternext_flags is called on a tied hash, the hash entry (HE)
that it returns has no value.  Perl_hv_copy_hints_hv, added in commit
5b9c067131, was assuming that it would have a value and calling
sv_magic on it, resulting in a crash.

Commit b50b205 made namespace::clean’s test suite crash, because
strict.pm started using %^H.  It was already possible to crash
namespace::clean with other hh-using pragmata, like sort:

    # namespace::clean 0.21 only uses ties in the absence of B:H:EOS
    use Devel::Hide 'B::Hooks::EndOfScope';
    use sort "stable";
    use namespace::clean;
    use sort "stable";
    {;}

It was possible to trigger the crash with no modules like this:

    package namespace::clean::_TieHintHash;

    sub TIEHASH  { bless[] }
    sub STORE    { $_[0][0]{$_[1]} = $_[2] }
    sub FETCH    { $_[0][0]{$_[1]} }
    sub FIRSTKEY { my $a = scalar keys %{$_[0][0]}; each %{$_[0][0]} }
    sub NEXTKEY  { each %{$_[0][0]} }

    package main;

    BEGIN {
	$^H{foo} = "bar";
	tie( %^H, 'namespace::clean::_TieHintHash' );
	$^H{foo} = "bar";
    }
    { ; }

This commit puts in a simple null check before calling sv_magic.  Tied
hint hashes still do not work, but they now only work as badly as in
5.8 (i.e., they don’t crash).

I don’t think tied hint hashes can ever be made to work properly, even
if we do make Perl_hv_copy_hints_hv copy the hash properly, because in
the scope where %^H is tied, the tie magic takes precedence over hint
magic, preventing the underlying he chain from being updated.  So
hints set in that scope will just not stick.