MAPREDUCE-3953. [Gridmix] Gridmix throws NPE and does not simulate a job if the trace contains null taskStatus for a task.  (ravigummadi via tgraves)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1461242 13f79535-47bb-0310-9956-ffa450edef68