Fix e83f24e3a6 and 1ff20e8f6a miniupnpc leak and crash on thread exit.

std::thread crashes when destroyed without being joined or detached.
Joining would wait on the UPnP thread, which can take upwards of 10
seconds, so detach instead.

Reviewed By: elexis
Differential Revision: https://code.wildfiregames.com/D2181
This was SVN commit r22674.

source/network/NetServer.cpp

@@ -159,6 +159,11 @@ CNetServerWorker::~CNetServerWorker()
 		m_WorkerThread.join();
 	}
 
+#if CONFIG2_MINIUPNPC
+	if (m_UPnPThread.joinable())
+		m_UPnPThread.detach();
+#endif
+
 	// Clean up resources
 
 	delete m_Stats;