Make sure a passthrough client is still in the hash table before deleting it

src/stratifier.c

@@ -3017,6 +3017,7 @@ static void parse_method(sdata_t *sdata, const int64_t client_id, json_t *id_val
 		 * Remove this instance since the client id may well be
 		 * reused */
 		ck_wlock(&sdata->instance_lock);
+		if (likely(__instance_by_id(sdata, client_id)))
 			HASH_DEL(sdata->stratum_instances, client);
 		ck_wunlock(&sdata->instance_lock);