In newATTRSUB, clear glob slot before lowering refcount.

In newATTRSUB, clear glob slot before lowering refcount.

(Actually in its subroutine, S_already_defined.)

Otherwise, when newATTRSUB redefines a sub, the previous sub’s DESTROY
can see the same sub still in the typeglob, but without a reference
count, so *typeglob = sub {} frees the sub currently in $_[0].

$ perl5.18.1 -le '
    sub foo{}
    bless \&foo;
    DESTROY {
        print "before: $_[0]"; *foo=sub{}; print "after: $_[0]"
    }
    eval "sub foo{}";
'
before: main=CODE(0x7fa88382d6d8)
before: main=CODE(0x7fa88382d6d8)
after: main=CODE(0x7fa88382d6d8)
after: UNKNOWN(0x7fa88382d6d8)