IMPALA-1456: Hive UDFs with String args would crash impalad

The wrong buffer was being used.

Change-Id: I18bf9040eaeda871d1d0baee2e276749a3a38615
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5185
Reviewed-by: Casey Ching <casey@cloudera.com>
Tested-by: jenkins