Continuation of 8028d717

Bug 5165

It was still possible for job priority to go down to 0, because I was
letting last_prio get set down to 1, then later on we were using
last_prio-1 as the new job priority. Since last_prio was 1, then new
job priority was 1-1=0. This happened when the job priorities were all
at 2 (which I never tested). Michael found it by running test2.26 in a
case where initial job priorities were 2:

spawn /home/marshall/slurm/18.08/voyager/bin/squeue --name=test2.26 -o JOB_ID=%A PRIO=%Q
JOB_ID=JOBID PRIO=PRIORITY
JOB_ID=10702 PRIO=2
JOB_ID=10703 PRIO=2
JOB_ID=10704 PRIO=2
Reset and test job priorities

spawn /home/marshall/slurm/18.08/voyager/bin/scontrol top 10704,10703
spawn /home/marshall/slurm/18.08/voyager/bin/squeue --name=test2.26 -o JOB_ID=%A PRIO=%Q
JOB_ID=JOBID PRIO=PRIORITY
JOB_ID=10704 PRIO=2
JOB_ID=10703 PRIO=1
JOB_ID=10702 PRIO=0
Reset and test job priorities