Fix pg_dump assertion failure when dumping pg_catalog.
Commit
396d348b04 did not account for the default collation.
Also, use pg_log_warning() instead of Assert().
Discussion: https://postgr.es/m/
ce071503fee88334aa70f360e6e4ea14d48305ee.camel%40j-davis.com
Reviewed-by: Michael Paquier
Backpatch-through: 15