job array update results in bad task ID

Fix to job array update logic that can result in a task ID of 4294967294.
To reproduce:
$ sbatch --exclusive -a 1,3,5 tmp
Submitted batch job 11825
$ scontrol update jobid=11825_[3,4,5] timelimit=3
$ squeue
             JOBID PARTITION     NAME     USER ST       TIME  NODES NODELIST(REASON)
           11825_3     debug      tmp    jette PD       0:00      1 (None)
           11825_4     debug      tmp    jette PD       0:00      1 (None)
           11825_5     debug      tmp    jette PD       0:00      1 (None)
             11825     debug      tmp    jette PD       0:00      1 (Resources)
A new job array entry was created for task ID 4 and the "master" job
array record now has a task ID of 4294967294.
The logic with the bug was using the wrong variable in a test.
bug 1790