Fix missing clearing of oc->busyobj on HSH_Fail.

The oc's busyobj isn't cleared in HSH_Fail (like it is in HSH_Complete).
This allows a thread to get a pointer to an already free'd busyobj.

Fixes: #1467

bin/varnishd/cache/cache_fetch.c

@@ -790,6 +790,7 @@ vbf_fetch_thread(struct worker *wrk, void *priv)
 				assert(uu == bo->fetch_obj->len);
 		}
 	}
+	AZ(bo->fetch_objcore->busyobj);
 
 	if (bo->ims_obj != NULL)
 		(void)HSH_DerefObj(&wrk->stats, &bo->ims_obj);

bin/varnishd/cache/cache_hash.c

@@ -642,6 +642,7 @@ HSH_Fail(struct objcore *oc)
 
 	Lck_Lock(&oh->mtx);
 	oc->flags |= OC_F_FAILED;
+	oc->busyobj = NULL;
 	Lck_Unlock(&oh->mtx);
 }