This error is displayed anytime the SELECT statement needs a temp table to
return correct results because the object (select_dumpvar) that represents
variables named in the INTO clause stored the results before the temp
table was considered.  The problem was fixed by creating the necessary
Item_func_set_user_var objects once the correct data is ready.


mysql-test/r/distinct.result:
  Bug#20836 Selecting into variables results in wrong results being returned
  - Added results
mysql-test/t/distinct.test:
  Bug#20836 Selecting into variables results in wrong results being returned
  - Added various Selects that use the INTO statement and a temp table.
  - Added Select Into Outfile variant tests also.
sql/sql_class.cc:
  Bug#20836 Selecting into variables results in wrong results being returned
  - The select_dumpvar variable created a Item_func_set_user_var too early
  and once set, it was not possible to change.  The Item_func_set_user_var
  is now created once the final results are available.
sql/sql_class.h:
  Bug#20836 Selecting into variables results in wrong results being returned
  - Removed unnecessary object members.