Never schedule the last task in a job array twice

The last task reuses array_job_id. So if job_ptr doesn't change, that
means we have already scheduled it and should never go to next_task.

job_launch() can requeue the job if it fails. This puts the job in the
completing and pending state, which could allow the last task to get
scheduled *again* without the check added here. Rescheduling a
completing job will destroy its node_bitmap and job_resrcs and cause
the job to stay completing forever.

Bug 10980

Co-authored-by: Brian Christiansen <brian@schedmd.com>