[perl #87664] Don’t autovivify stashes when anonymising CVs

[perl #87664] Don’t autovivify stashes when anonymising CVs

This commit stops CV anonymisation from autovivifying stashes to point
to (unless the stash is %__ANON__::).

If a stash has been deleted from its original position in the symbol
table, then its HvNAME will no longer indicate where to find it.

S_anonymise_cv_maybe in sv.c was using the HvNAME to look up (and
autovivify) the *__ANON__ glob in the stash, without taking into
account that it might not actually be looking in the right spot.

So now, after checking that the stash still has a name (HvNAME), it
uses the HvENAME to find it. If the HvENAME is null, which indicates
that the stash has been detached altogether, then %__ANON__:: is used,
as happens when HvNAME is null.

This solves a Class::Monadic failure introduced by commit 2d0d1eccfc
([perl #79208] %stash:: = () anonymises CVs), which was included
in 5.13.7.

Basically, it can be reduced to this: