Fix buffer overflow introduced in #44. (#71)

PR #44 introduced a buffer overflow bug that will corrupt the heap if the transmit payload size is increased after any send.

Co-authored-by: Dr. Fred Nugen <[email protected]>

Author: aentinger

src/MqttClient.cpp

@@ -802,6 +802,12 @@ void MqttClient::setConnectionTimeout(unsigned long timeout)
 
 void MqttClient::setTxPayloadSize(unsigned short size)
 {
+  if (_txPayloadBuffer) {
+    free(_txPayloadBuffer);
+    _txPayloadBuffer = NULL;
+    _txPayloadBufferIndex = 0;
+  }
+    
   _tx_payload_buffer_size = size;
 }