The problem here is that we try to evaluate a subquery from the
resolver. Item_is_not_null_test::update_used_tables() attempts
to call is_null() on its argument because with_subselect is
erroneously false. This field is wrongly updated because
Item_func_make_set::update_used_tables() has a bad propagation of
with_subselect: It first calls Item_func::update_used_tables() to update
data for each arguments, then it calls item->update_used_tables().
But instead of accumulating data for with_subselect and
with_stored_program, it assigns directly to those fields.

The fix is to convert the assignments into OR assignments.