Fix MQTT client: clear credentials and will on connection teardown

[EdouardMALOT]

Summary

The credentials and will pointers stored in the client (nxd_mqtt_client_username / password / will_topic / will_message, set by nxd_mqtt_client_login_set() and nxd_mqtt_client_will_message_set()) must be cleared on every connection teardown, error paths included. Today only the clean-disconnect path clears them, in _nxd_mqtt_process_disconnect(); the failed-connect teardowns (CONNACK refused, TCP or TLS establish failure) go through _nxd_mqtt_client_connection_end() and keep the stale pointers. If the application then reconfigures the same client for an anonymous connection (no new login_set() call) and its old buffers have been zeroed or reused, the next CONNECT goes out with the username flag set but no valid username. Mosquitto drops it as a protocol error (CONNECT with username flag but no username) and nxd_mqtt_client_secure_connect() fails with NXD_MQTT_COMMUNICATION_FAILURE (0x10007), on every retry, until the device reboots.

Observed during TLS/MQTT test-bench runs: connect attempts with credentials failed while the server certificate was temporarily invalid, the configuration was then switched back to an anonymous account, and the live client could never reconnect.

Changes

• addons/mqtt/nxd_mqtt_client.c
  • Move the username/password/will cleanup from _nxd_mqtt_process_disconnect() into _nxd_mqtt_client_connection_end(), so every teardown path resets it. _nxd_mqtt_process_disconnect() calls _nxd_mqtt_client_connection_end(), so the clean-disconnect behavior is unchanged.
  • Apply the same cleanup in the early TCP-connect-failure branch of _nxd_mqtt_client_connect(), the only teardown that does not go through _nxd_mqtt_client_connection_end(). Also clear nxd_mqtt_client_use_tls there: that branch has the same stale-flag problem that Fix MQTT client: reset use_tls flag in connection_end (TLS to plain reconnect) #400 fixes in _nxd_mqtt_client_connection_end(), on a path Fix MQTT client: reset use_tls flag in connection_end (TLS to plain reconnect) #400 does not cover. (The two PRs touch different hunks and merge independently.)

Behavioral note: credentials were already cleared on every clean disconnect by _nxd_mqtt_process_disconnect(), so the effective contract is already "call login_set() before each connect that needs credentials". This change only extends the existing cleanup to the failure paths.

Test plan

• Reproduced on mosquitto: after a failed connect with credentials, an anonymous reconnect on the same client sends CONNECT with the username flag and no username, and is dropped by the broker as a protocol error
• Audit of all teardown call sites: _nxd_mqtt_process_disconnect, _nxd_mqtt_process_connack error path, TCP/TLS establish-process failures, and the early TCP-failure branch of _nxd_mqtt_client_connect all reset the credentials with this change
• Full device-level regression on our gateway test bench with the patched library, including secure and plain connect/disconnect/reconnect cycles with credentials re-set before each connect (in progress)