IMPALA-906: Fix bug in tracking of row batch auxiliary memory.

We were clearing the mem pool before updating the counter.

Change-Id: Ifd1cfa0ffc28234d403471cc5cb22ac2d4e41091
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2023
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins