BUG #8405: user can create a system table(eg, pg_class)

The following bug has been logged on the website:

Bug reference:      8405
Logged by:          xjxia
Email address:      syxjxia@gmail.com
PostgreSQL version: 9.2.2
Operating system:   ubuntu11.04 x86_64
Description:

postgres=# create table pg_class(a int);
CREATE TABLE
postgres=# insert into pg_class values (1);
ERROR:  null value in column "relnamespace" violates not-null constraint
DETAIL:  Failing row contains (1, null, null, null, null, null, null, null,
null, null, null, null, null, null, null, null, null, null, null, null,
null, null, null, null, null, null, null).
postgres=#