SUBSELECT_UNION_ENGINE::NO_ROWS

This patch is specific for mysql-5.5

ISSUE: When max_join_size is used and union query
       results in evaluation of tuples greater than
       max_join_size, the join object is not created,
       and is set to NULL.
       However, this join object is further dereferenced
       by union logic to determine if query resulted in
       any number of rows being returned.
       Since, the object is NULL, it results in
       program terminating abnormally.

SOLUTION: Added check to verify if join object is created.
          If join object is created, it will be used to
          determine if query resulted in any number of rows.
          Else, when join object is not created, we return
          'false' indicating that there were no rows for the
          query.