Fix Null Reference Exception on stopping Quartz

When other code uses a BeforeStartingService
but cancels service start, this extensions
BeforeStoppingService is called without the
BeforeStartingService ever being called. In this
situation, Scheduler is null, and calling
Scheduler.Shutdown() causes a Null Reference Exception.

```
// Other TopShelf startup code.
s.BeforeStartingService(hsc =>
  {
	if (CanStartPreconditionsMet())
	{
	    return;
	}

    Log.Fatal("Some message.");

    hsc.CancelStart();
});
```