JCR-3483 nullify current node element before session is alive check is made

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/branches/2.2@1421478 13f79535-47bb-0310-9956-ffa450edef68
JCR-3486 guard against npe due to transient root being null

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/branches/2.2@1421857 13f79535-47bb-0310-9956-ffa450edef68