Re: fix NOT VALID NOT NULL with ALTER COLUMN SET IDENTITY

Pushed now to 18 and master.  Thanks.

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
"Once again, thank you and all of the developers for your hard work on
PostgreSQL.  This is by far the most pleasant management experience of
any database I've worked on."                             (Dan Harris)
http://archives.postgresql.org/pgsql-performance/2006-04/msg00247.php