IMPALA-1111: Fix alignment in ReservoirSample aggregate functions

Change-Id: Iac7aa96eb19079715a7e8152a5edfeafa0d50bc7
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4478
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins