Fixed bug #6187: a wrong initial setting for const_item_cache
  in Item_cond::fix_fields.


sql/item_cmpfunc.cc:
  Fixed bug #6187: a wrong initial setting for const_item_cache
  in Item_cond::fix_fields.
mysql-test/r/func_test.result:
  Fixed bug #6187: a wrong initial setting for const_item_cache
  in Item_cond::fix_fields.