Fix possible crash in ALTER TABLE ... REPLICA IDENTITY USING INDEX.

Careless coding added by commit 07cacba983ef79be could result in a crash
or a bizarre error message if someone tried to select an index on the
OID column as the replica identity index for a table.  Back-patch to 9.4
where the feature was introduced.

Discussion: CAKJS1f8TQYgTRDyF1_u9PVCKWRWz+DkieH=U7954HeHVPJKaKg@mail.gmail.com

David Rowley
