Move the check_new() test to the start of sv_size()

Move the check_new() test to the start of sv_size()

For now, this requires sv_size() to return new-or-not, but it prepares the way
to move all structure recursion into sv_size(). Currently it's in two places -
sv_size() and total_size().

This resolves two long-standing bugs in typeglobs - total_size() was double
counting entries in typeglobs, and sv_size() was double-counting the PVGV size
if GvEGV() looped back.