In Perl_pad_add_name(), use sv_upgrade() directly rather than new[AH]V().

In Perl_pad_add_name(), use sv_upgrade() directly rather than new[AH]V().

As newAV() and newHV() are now merely wrappers around sv_upgrade(), and the
existing SV is always brand new and of type SVt_NULL, call them on it, rather
than disposing of it as a side effect of storing a(nother new) SV.

Also, no need to set SvPADMY() again, as it is already set.

Resolves RT #73092.