[ofw] Add Assert to debug BS while deleting WDF device

[Hefty, Sean]

> Okay. I get it. I'm not sure it's required but I can live with that.
> 
> However I want to keep the ASSERT so in running time we can check the
> assumptions. I made some changes in the code please review.

The patch looks like it will fix the blue screen.  I will still see if the locking can be simplified.

Something still needs to be done to see if we can avoid the failure from WdfDeviceCreateSymbolicLink.

- Sean