No need to explicitly check AvARYLEN in 5.10 and later.

No need to explicitly check AvARYLEN in 5.10 and later.

In 5.10 the struct slot was eliminated, and contents are now stored in magic,
which will already be found. Whilst there's no direct "harm" in looking again,
as the "seen" tracker will skip the second discovery, it's wasteful, and now
generates a compilation error, as AvARYLEN() expects a mutable pointer, whereas
the variable thing is const void *.

This also resolves CPAN RT #49437 - Devel::Size adds magic in Perl 5.10