Arg_comparator initializes 'comparators' array in case of
ROW comparison and does not free this array on destruction.
It leads to memory leaks.
The fix:
-added Arg_comparator::cleanup() method which frees
 'comparators' array.
-added Item_bool_func2::cleanup() method which calls 
 Arg_comparator::cleanup() method


mysql-test/r/ps.result:
  test case
mysql-test/r/row.result:
  test case
mysql-test/t/ps.test:
  test case
mysql-test/t/row.test:
  test case
sql/item_cmpfunc.h:
  -added Arg_comparator::cleanup() method which frees
   'comparators' array.
  -added Item_bool_func2::cleanup() method which calls 
   Arg_comparator::cleanup() method