Verify the entire region trailer, not just its offset, is within data area

Verify the entire region trailer, not just its offset, is within data area

- Offset being within the data area doesn't help if the actual data doesn't
  fit. Since the trailer size is well known, we can just as easily
  make the check accurate to prevent reading beyond end of data in case
  the offset is subtly wrong.
- In headerLoad(), region offset of zero doesn't need sanity checking,
  only validate if its something else and do so accurately there too.