JobTracker.setJobPriority() doesn't check for a jobID lookup failing

Looking at the entry points of the JobTracker API, it seems that JobTracker.setJobPriority() doesnt expect the JobID lookup ever to return null

It goes straight from lookup to checking access, an operation that assumes that job!=null
JobInProgress job = jobs.get(jobid);
checkAccess(job, QueueManager.QueueOperation.ADMINISTER_JOBS);

Recommend: add a test that calls this operaton with an invalid jobID, then fix the code as appropriate