aggregate function reinitialization (if no rows was found) made with help of clear()
(fixed BUG#860)


mysql-test/r/subselect.result:
  test for BUG#860
mysql-test/t/subselect.test:
  test for BUG#860
sql/item_sum.cc:
  reset() replaced with clear()
sql/item_sum.h:
  reset() replaced with clear()
  (reset now is just composition of clear() and add())
sql/item_uniq.h:
  reset() replaced with clear()
sql/sql_select.cc:
  removed NULL value assigment, now it will be done by clear() call
  function for clearing if there was not found any row in group
sql/sql_select.h:
  function for clearing if there was not found any row in group