End the client thread when returns to welcome scene

Fix a bug when returns to the welcome scene the client thread is still running, and this can even cause the player to match with him/her self because the old thread is still running.

Author: CharmySoft

ttt_client_gui.py

@@ -536,6 +536,9 @@ def __on_return_clicked__(self):
 button is clicked."""
 # Clear screen
 self.__clear_screen();
+# Set the client to None so the client thread will stop due to error
+self.client.client_socket = None;
+self.client = None;
 # Switch to the welcome scene
 self.pack_forget();
 self.welcome_scene.pack();