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.