statement.

The problem was that during statement re-execution if the result was
empty the old result could be returned for group functions.

The solution is to implement proper cleanup() method in group
functions.


mysql-test/r/ps.result:
  Add result for bug#21354: (COUNT(*) = 1) not working in SELECT inside
  prepared statement.
mysql-test/t/func_gconcat.test:
  Add a comment that the test case is from bug#836.
mysql-test/t/ps.test:
  Add test case for bug#21354: (COUNT(*) = 1) not working in SELECT inside
  prepared statement.
sql/item_sum.cc:
  Call clear() in Item_sum_count::cleanup().
sql/item_sum.h:
  Add comments.
  Add proper cleanup() methods.
  Change Item_sum::no_rows_in_result() to call clear() instead of reset(),
  as the latter also issues add(), and there is nothing to add when there
  are no rows in result.