[perl #115260] Stop length($obj) from returning undef

[perl #115260] Stop length($obj) from returning undef

When commit 9f621bb00 made length(undef) return undef, it also made it
return undef for objects with string overloading that returns undef.

But stringifying as undef is a contradiction in terms, and this makes
length inconsistent with defined, which returns true for such objects.

Changing this allows is to simplify pp_length, as we can now call
sv_len_utf8 on the argument unconditionally (except under the bytes
pragma).  sv_len_utf8 is now careful not to record caches on magical
or overloaded scalars (any non-PV, in fact).

Note that sv_len is now just a wrapper around SvPV_const, so we use
SvPV_const_nomg, as there is no equivalent sv_len_nomg.