This bug happens when a query, having subselects in the fields list,
needs temporary table.
Item_subselect::get_tmp_table_item isn't implemented and just returns
*this. So the tmp_table_item takes value not from temporary but
from original table.


mysql-test/r/subselect.result:
  appropriate test result added
mysql-test/t/subselect.test:
  test case added
sql/item_subselect.cc:
  Item_subselect::get_tmp_table_item implementation
sql/item_subselect.h:
  Item_subselect::get_tmp_table_item declaration