Fixed NullPointerException in MesosScheduler when using the Hadoop
Fair Scheduler as the underlying scheduler.

From: Guodong Wang <wanggd04@gmail.com>
Review: https://reviews.apache.org/r/10695

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1471072 13f79535-47bb-0310-9956-ffa450edef68