At the end of execution top level join execution
we cleanup this join with true argument.
It leads to underlying join cleanup(subquery) with true argument too
and to tmp_table_param->field array cleanup which is required later.
The problem is that Item_func_set_user_var does not set
result_filed which leads to unnecessary repeated excution of subquery
on final stage.
The fix is to set result_field for Item_func_set_user_var.


mysql-test/r/count_distinct.result:
  test result
mysql-test/r/user_var.result:
  test result
mysql-test/t/count_distinct.test:
  test case
mysql-test/t/user_var.test:
  test case
sql/item_func.cc:
  At the end of execution top level join execution
  we cleanup this join with true argument.
  It leads to underlying join cleanup(subquery) with true argument too
  and to tmp_table_param->field array cleanup which is required later.
  The problem is that Item_func_set_user_var does not set
  result_filed which leads to unnecessary repeated excution of subquery
  on final stage.
  The fix is to set result_field for Item_func_set_user_var.