Item_in_subselect's only externally callable method is val_bool().
However the nullability in the wrapper class (Item_in_optimizer) is 
established by calling the "forbidden" method val_int().

Fixed to use the correct method (val_bool() ) to establish nullability 
of Item_in_subselect in Item_in_optimizer.


mysql-test/r/subselect.result:
  Bug #31884: test case
mysql-test/t/subselect.test:
  Bug #31884: test case
sql/item_subselect.h:
  Bug #31884: Use the correct method to establish nullability