Fix memtracker instantiation when using FETCH_FIRST

Change-Id: I47b614b3559880f428951b015291bee4f5af6c49
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3038
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins
(cherry picked from commit 1d34f23d69dc1a55e1e68a8acae94a57945f86e0)
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3256