Fix memtracker instantiation when using FETCH_FIRST
Fix memtracker instantiation when using FETCH_FIRST

Change-Id: I47b614b3559880f428951b015291bee4f5af6c49
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3038
Reviewed-by: Henry Robinson <3eca10f30041813f045165784e24b5a950a6cc7e@cloudera.com>
Tested-by: jenkins

cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala