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.

mysql-test/r/subselect3.result:
  Added test case for BUG#48920
mysql-test/t/subselect3.test:
  Added test case for BUG#48920
sql/item_sum.cc:
  If an aggregator exists when Item_sum::set_aggregator() is
  called (i.e., set_aggregator is called in a dependent
  subquery), the aggregator is reset so that the aggregator is
  reevaluated with the dependent information from the outer
  record being evaluated.