IMPALA-1213: Temporarily remove DCHECK and fix computation of rows ready to return

Change-Id: I25587e38d6ac22da58b2f68bb68aac5228c22bbd
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4227
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: Matthew Jacobs <mj@cloudera.com>