used.

The Item::save_in_field() function is called from fill_record() to fill the 
new row with data while execution of the CREATE TABLE ... SELECT statement.
Item::save_in_field() calls val_xxx() methods in order to get values.
val_xxx() methods do not take into account the result field. Due to this
Item_func_set_user_var::val_xxx() methods returns values from the original
table, not from the temporary one.

The save_in_field() member function is added to the Item_func_set_user_var
class. It detects whether the result field should be used and properly updates
the value of the user variable.


sql/item_func.cc:
  Bug#16861: User defined variable can have a wrong value if a tmp table was used.
  Added the save_in_field() member function to the Item_func_set_user_var class.
sql/item_func.h:
  Bug#16861: User defined variable can have a wrong value if a tmp table was used.
  Added the save_in_field() member function to the Item_func_set_user_var class.
mysql-test/r/user_var.result:
  Extended the test case for bug#18681: User defined variable can have a wrong value if
  a tmp table was used.
mysql-test/t/user_var.test:
  Extended the test case for bug#18681: User defined variable can have a wrong value if
  a tmp table was used.