Avoids a dangling pointer.

When doKill is called on KGetJobAdapter or KGetGlobalJob requestStop(KJob*, TransferHandler*) is emitted.

KUiServerJob reacts on that:
*unregisters the job if it was registered
*stops the transfer or all transfers if transfer is 0.

That way the job is unregistered before its KWidgetJobTracker is destroyed avoiding a crash.