Inplace update assert after instant adding column

In MariaDB 10.3.2, instant adding columns for innodb is supported.
After instant adding columns, the inplace update trigger assertion in some cases.

How to repeat:
create table t1 (c1 int primary key, c2 int);
insert into t1 values(1,1);
alter table t1 add column d1 varchar(20) not null default 'aaa';
update t1 set d1 = '' where c1 = 1; – assert in rec_set_nth_field()

The length of "d1" stored in the record is 0, because of instant adding columns. After update, the length is 0 also. But it can't do inplace update here.

So, it need to do some special check for rec_offs_nth_default() in row_upd_changes_field_size_or_external();

The patch can be work for it.