assume cleared hv can't be re-blessed

assume cleared hv can't be re-blessed

followup to previous commit. I'm fairly confident now that a HV being
freed in sv_clear() can never get re-blessed, and thus its SvSTASH field is
*always* safe to use.

The logic behind this is that once you get to sv_clear(), the HV has a
refcnt of zero, which means either:

* nothing references this, so there's no way to bless it;
* we're in SVf_BREAK territory, in which case something may still
  hold a reference to it, but all destructors will have already been
  called, so nothing can call bless.