Prevent NPE in deallocate_pages (#688)

Prevent NPE in `deallocate_pages`

If `deallocate_pages` is called on a job for which `allocate_pages`
has not been called (see `iterate_start_jobs` for conditions under
which this is true), `allocated_pages` is `None`, raising a NPE
when attempting to iterate.

In particular, this prevents `clear_queue` from working. In
practice, this problem readily occurs when starting a few jobs
and then calling `clear_queue`.