Fixed killtree to guard against session ids being reported as '0'
by '/bin/ps' on OSX 10.8.

Review: https://reviews.apache.org/r/9915

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1456300 13f79535-47bb-0310-9956-ffa450edef68