Fixes a memory leak / message decoding error when a MQTT client disconnects.

CollectAgent/simplemqttserverthread.cpp

@@ -375,6 +375,11 @@ void SimpleMQTTServerMessageThread::releaseConnection(int connectionId)
   fds[connectionId].events = 0;
   fds[connectionId].revents = 0;
 
+  if(msg[connectionId]) {
+    delete msg[connectionId];
+    msg[connectionId] = NULL;
+  }
+
   numConnections--;
   fdsMtx.unlock();