Proper memory management for QHttpServer::m_tcpServer

Should solve a memory leak. 
Trying to listen on different ports would overwrite m_tcpServer with a new value without deleting the old one and still leave the SIGNAL(newConnection()) connection intact.

Now m_tcpServer will only setup connections when m_tcpServer->listen() was successful, otherwise it will be deleted and NULLed.
The destructor of QHttpServer will now close and delete m_tcpServer.