Add assertion to hopefully help coverity understand the code

Ref CID#469268

src/fellow_cache.c

@@ -3207,6 +3207,7 @@ fellow_cache_lru_work(struct worker *wrk, struct fellow_cache_lru *lru)
 		if (fcs->state == FCO_INCORE) {
 			oc = fco->oc;
 			CHECK_OBJ_NOTNULL(oc, OBJCORE_MAGIC);
+			assert(oc->stobj->priv == fco);
 			AZ(fcs->fco_lru_mutate);
 			fcs->fco_lru_mutate = 1;