IMPALA-1066: Fix bad free in Min()/Max() of strings.

Change-Id: If66844a88accdc369458ab92f033eef50775d69e
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3363
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins