NetPlay: Make sure the server knows it's stopped when it's stopped

This is embarassing.

Source/Core/Core/Src/NetPlayServer.cpp

@@ -581,6 +581,8 @@ bool NetPlayServer::StopGame()
 	std::lock_guard<std::recursive_mutex> lks(m_crit.send);
 	SendToClients(spac);
 
+	m_is_running = false;
+
 	return true;
 }