Patch: Yahoo Transport logoff when signing in on other device

There is a small bug in gateway that will prevent the Yahoo transport from being disconnected when the user signs in using Yahoo on somer other computer. The gateway will still show Yahoo as connected even though the user has been disconnected and cannot send or receive any messages.

The proposed change for

public void connectionClosed(SessionEvent event) {
        // PATCH: do not log, event may be null           // Log.debug(event.toString());
        if (getSession().isLoggedIn()) {

In case of a Yahoo LOGOFF event the event parameter is null in which case this function will throw a NullPointerExcpetion.