You need to comment MQTT_VAR_HEADER_BUFFER_LEN assert in mqtt.c
,because it's error.
msg_idx can be more or equeal MQTT_VAR_HEADER_BUFFER_LEN, becouse it's handled by a callback function.
// error ASSERT in LwIP 2.1.1
//LWIP_ASSERT("client->msg_idx < MQTT_VAR_HEADER_BUFFER_LEN", client->msg_idx < MQTT_VAR_HEADER_BUFFER_LEN);