In newXS, clear glob slot before lowering refcount.

In newXS, clear glob slot before lowering refcount.

Otherwise, when newXS 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 re::regmust{}
    bless \&re::regmust;
    DESTROY {
        print "before: $_[0]"; *re::regmust=sub{}; print "after: $_[0]"
    }
    require re;
'
before: main=CODE(0x7ff7eb02d6d8)
before: main=CODE(0x7ff7eb02d6d8)
after: main=CODE(0x7ff7eb02d6d8)
after: UNKNOWN(0x7ff7eb02d6d8)