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.

```c#
// Other TopShelf startup code.
s.BeforeStartingService(hsc =>
  {
	if (CanStartPreconditionsMet())
	{
	    return;
	}

    Log.Fatal("Some message.");

    hsc.CancelStart();
});
```