Add clarifying assertion

we only jump to the again label if we did not get a reference.

src/fellow_cache.c

@@ -4780,7 +4780,7 @@ struct objcore **ocp, uintptr_t priv2, unsigned crit)
 	CAST_OBJ_NOTNULL(nfco, fcr.r.ptr, FELLOW_CACHE_OBJ_MAGIC);
 
   again:
-
+	AZ(oref);
 	//lint --e{456} flexelint does not grok trylock
 	AZ(pthread_mutex_lock(&fc->fdb_mtx));
 	fco = VRBT_INSERT(fellow_cache_fdb_head, &fc->fdb_head, nfco);