pg_catalog: fix up pg_catalog.pg_attribute

[yuzefovich]

In postgres, the following query returns 0 rows:

-- Cross-check attnum against parent relation

SELECT a1.attrelid, a1.attname, c1.oid, c1.relname
FROM pg_attribute AS a1, pg_class AS c1
WHERE a1.attrelid = c1.oid AND a1.attnum > c1.relnatts;

while we receive a few hundred. We should fix it up (and adjust pg_regress roachtest accordingly).

Jira issue: CRDB-38492