MDEV-15746 ASAN heap-use-after-free in Item_change_list::rollback_item_tree_changes on ALTER executed as PS

10.3+ fix

On ALTER TABLE, if a non-changed column default
might need a charset conversion, it must
be a blob. Because blob's defaults ar stored
as expressions, and for any other type
a basic_const_item() will be in the record,
so it'll have correct charset and won't need
converting. For the same reason it makes no
sense to convert blob defaults (and it's
unsafe, see MDEV-15746).

test case is already in main/ps.test