When the do..while loop in HSH_Purge executes on a oh with many
popular variants, there is a potential problem with the "array" of oc
pointers, allocated in the thread workspace. If many of the oc's have
positive refcounts, they will fill up the array and

	EXP_Rearm(oc, now, ttl, grace, keep);
	(void)HSH_DerefObjCore(wrk, &oc, 0);

will be called several on the same oc's. At the same time, the counter
n_obj_purged will be updated with a too low number.

The test case demonstrates how we get a too low value for this counter,
but it is not able to force varnishd to use a siginificant amount of CPU.