Multivariate MCV lists -- pg_mcv_list_items() seems to be broken

I just noticed the following:

CREATE TABLE foo (a int, b int);
INSERT INTO foo SELECT x/10, x/100 FROM generate_series(1, 100) x;
CREATE STATISTICS foo_s ON a,b FROM foo;
ANALYSE foo;

SELECT pg_mcv_list_items(stxmcv) from pg_statistic_ext WHERE stxname = 'foo_s';

which fails with

ERROR: cache lookup failed for type 0

That definitely used to work, so I'm guessing it got broken by the
recent reworking of the serialisation code, but I've not looked into
it.

There should probably be regression test coverage of that function.

Regards,
Dean