add tx buffer variable in constructor

Author: Rocketct

src/MqttClient.cpp

@@ -70,6 +70,9 @@ MqttClient::MqttClient(Client& client) :
   _connected(false),
   _subscribeQos(0x00),
   _rxState(MQTT_CLIENT_RX_STATE_READ_TYPE),
+  _txBufferIndex(0),
+  _txPayloadBuffer(NULL),
+  _txPayloadBufferIndex(0),
   _willBuffer(NULL),
   _willBufferIndex(0),
   _willMessageIndex(0),