IMPALA-1293: Fix DCHECK failure with window ROWS BETWEEN UNBOUNDED PRECEDING

Change-Id: I4e92e9593402f4341826c6940e23e493c7d23641
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4487
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins