Fix crash when lex subs are used for AUTOLOAD
Fix crash when lex subs are used for overload
Fix crash in leave_scope when my sub has CvGV

Sub declaration can reuse an existing stub.  So it is possible to define
a package sub using a stub that was originally lexical.  Hence,
leave_scope needs to take into account that a my-sub may not have a
name hek any more.
Fix crash with lex subs under -d
[Merge] Fix crashes with lexical subs

Various crashing resulting from code not taking CvGV(lexsub)==NULL
into account.