Prevent wrong job array task ID from being shown

Prevent job array task ID from being reported as NO_VAL if last task in the
  array gets requeued. The problem is that when that task starts, the task
  bitmap entry for it stays set, but the task counter gets decremented.
  If that job then gets requeued, under some conditions a failure to schedule
  it results in the array_task_id in the job record getting set to NO_VAL.
  Then when building the job info to report for squeue/scontrol, the string
  showing the pending task ID's is not rebuilt due to that counter being
  zero. All indications are that the job runs fine, only the information
  reported to squeue/scontrol is wrong.
bug 1790