Properly terminate the XMPP connection

This ensures the XMPP connection gets properly terminated when leaving
the lobby.

Fixes #7504

source/lobby/XmppClient.cpp

@@ -169,6 +169,8 @@ XmppClient::XmppClient(const ScriptInterface* scriptInterface, const std::string
  */
 XmppClient::~XmppClient()
 {
+	this->disconnect();
+
 	DbgXMPP("XmppClient destroyed");
 	delete m_registration;
 	delete m_mucRoom;