ZOOKEEPER-1222. getACL should only call DataTree.copyStat when passed in stat is not null (Michi Mutsuzaki via rakeshr)

git-svn-id: https://svn.apache.org/repos/asf/zookeeper/branches/branch-3.4@1608873 13f79535-47bb-0310-9956-ffa450edef68