Restore of quiz module may create calendar events without a date/time

When/after a module is restored, the <mod>_refresh_events() is invoked if it exists. In the case of the quiz module this adds calendar events for the corresponding quiz dates. However, unfortunately it adds events for quizzes/dates that have been disabled, so a calendar event is created for the course with no starting time i.e. event.timestart = 0. To the best of my knowledge, the attached patch fixes this issue, by only created calendar events if the quiz has a start date/time.