IMPALA-1220: NPE in AnalyticWindow during cloning and thrift serialization

Change-Id: Id2d7aa1542a186da941b8c0b723ff5698243f5cd
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4278
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins