MDEV-28967 Assertion `marked_for_write_or_computed()' failed in Field_new_decimal::store_value / online_alter_read_from_binlog`

in the catch-up phase of the online alter we apply row events,
they're unpacked into `from->record[0]` and then converted
to `to->record[0]`.

This needs all fields of `from` to be in the `write_set`.

Although practically `Field::unpack()` does not assert the `write_set`,
and `Field::reset()` - used when a field value is not present in the
after-image - also doesn't assert the `write_set` for many types,
`Field_new_decimal::reset()` does.