Occassional assertion failure in multi_extension

When running regression tests locally, I occassionally see the following assertion failure in multi_extension:
```
TRAP: FailedAssertion("found", File: "utils/maintenanced.c", Line: 185)
```

in this part of the test:
```sql
-- check that maintenance daemon gets (re-)started for the right user
DROP EXTENSION citus;
CREATE USER testuser SUPERUSER;
SET ROLE testuser;
CREATE EXTENSION citus;

SELECT datname, current_database(),
usename, (SELECT extowner::regrole::text FROM pg_extension WHERE extname = 'citus')
FROM test.maintenance_worker();

-- and recreate as the right owner
RESET ROLE;
DROP EXTENSION citus;
```