subquery in the select list
      
When a dependent subquery with count(distinct <col>) was 
evaluated multiple times, the Distinct_Aggregator was reused. 
However, the Aggregator was not reset, so when the subquery was
evaluated for the next record in the outer select, old dependent
info was used.
      
The fix is to clear() the existing aggregator in 
Item_sum::set_aggregator(). This ensures that the aggregator is
reevaluated with the new dependent information.