Ok, it was a bad assumption on my part that the mapOutputTracker would only be returning valid locations, and this PR does address the possibility of null values here.  However, it feels to me like it is addressing the symptom instead of the cause.  Wouldn't we be better off either not putting nulls into the MapOutputTracker in the first place, or at least not returning them when getting map output statuses?  As it stands, null checks are probably needed everywhere the output statuses are fetched.