fix: send Malformed Packet CONNACK for username parse error in MQTT 5.0

[liooooo29]

Problem

When a MQTT 5.0 CONNECT packet has the Username Flag set but the username field is missing or contains invalid data, Mosquitto silently closes the connection without sending a CONNACK packet.

Per the MQTT 5.0 OASIS specification §3.1.2.11, if the Username Flag is set to 1, the Username must be present as a valid UTF-8 encoded string.

Current Behavior

// read_and_verify_client_credentials_from_packet() in handle_connect.c
if(username_flag){
    rc = set_username_from_packet(context, username, clientid);
    if(rc != MOSQ_ERR_SUCCESS){
        return rc;  // Connection closed without CONNACK
    }
}

Expected Behavior

Per [MQTT-3.1.2-11] and §4.13, the server should send a CONNACK with Reason Code 0x81 (Malformed Packet) before closing the connection.

Fix

if(username_flag){
    rc = set_username_from_packet(context, username, clientid);
    if(rc != MOSQ_ERR_SUCCESS){
        if(context->protocol == mosq_p_mqtt5){
            send__connack(context, 0, MQTT_RC_MALFORMED_PACKET, NULL);
        }
        return rc;
    }
}

Test Case

Verified with an automated MQTT 5.0 conformance test (T-92) that sends CONNECT with Username Flag=1 but no username data, and expects a CONNACK with Reason Code 0x81.

References

• MQTT 5.0 OASIS Standard (2019-03-07), §3.1.2.11: User Name Flag
• [MQTT-3.1.2-11]: If User Name Flag is set to 1, the User Name must be present