Added a test case for bug #9017.
item_cmpfunc.h:
  A wrong not_null_tables method for Item_cond_xor
  caused a conversion of a left join into an inner join
  that was not valid.


sql/item_cmpfunc.h:
  A wrong not_null_tables method for Item_cond_xor
  caused a conversion of a left join into an inner join
  that was not valid.
mysql-test/t/join_outer.test:
  Added a test case for bug #9017.
mysql-test/r/join_outer.result:
  Added a test case for bug #9017.