Add mosquitto_loop_stop before mosquitto_destroy in close()

The background network thread started by mosquitto_loop_start() was not
being stopped before mosquitto_destroy(), which can cause a use-after-free
if the thread is still running when the mosquitto structure is freed.

MQTTConnection.cpp

@@ -146,6 +146,7 @@ void CMQTTConnection::close()
 {
 	if (m_mosq != nullptr) {
 		::mosquitto_disconnect(m_mosq);
+		::mosquitto_loop_stop(m_mosq, true);
 		::mosquitto_destroy(m_mosq);
 		m_mosq = nullptr;
 	}