kiji-hive-adapter: KIJIHIVE-49. Fix resource leak in KijiRowDataWritable

issue: https://jira.kiji.org/browse/KIJIHIVE-49
review: https://review.kiji.org/r/1887
commit: https://github.com/kijiproject/kiji-hive-adapter/commit/b5f5cee6b1398c5d460af6bf05551e5554d74c29