cache lookup failed from empty plpythonu function

I've found a bug in plpythonu resulting in a "cache lookup" failure.
Here's the SQL to reproduce (thanks Andres):

CREATE EXTENSION plpythonu;
CREATE OR REPLACE FUNCTION t() RETURNS trigger AS 'pass' LANGUAGE 'plpythonu';
CREATE TABLE a();
CREATE TABLE b();
CREATE TRIGGER check_quota AFTER INSERT ON a FOR EACH ROW EXECUTE PROCEDURE t();
INSERT INTO a DEFAULT VALUES;
DROP TABLE a;
CREATE TRIGGER check_quota
AFTER INSERT ON b FOR EACH ROW EXECUTE PROCEDURE t();
INSERT INTO b DEFAULT VALUES;
--ERROR: cache lookup failed for relation 20200
--Time: 0.570 ms
-- cleanup with DROP TABLE b; DROP FUNCTION t();

Logs show:

ERROR: XX000: cache lookup failed for relation 3694414
LOCATION: PLy_procedure_argument_valid, plpython.c:1469

Andres (in IRC #postgresql) said:

<andres> hm, I think I see the problem. The caching there looks pretty
broken to me.
<andres> i.e. the caching doesn't work *at all* if a function is used
as a trigger for more than one table and this is just fallout
of that.
<andres> Looks like a typo to me, procedure_get should use the table's
oid instead of fn_oid when doing lookups in PLy_trigger_cache.

Tested on:

PostgreSQL 9.1.7 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3, 64-bit

It was probably also present in the stable version as of one year
ago ( see https://github.com/Vizzuality/cartodb/issues/173 )

--strk;