Segfault when creating partition with a primary key and sql_drop trigger exists

[Marco Slot]

We're seeing a segmentation fault when creating a partition of a
partitioned table with a primary key when there is a sql_drop trigger on
Postgres 11beta4.

We discovered it because the Citus extension creates a sql_drop trigger,
but it's otherwise unrelated to the Citus extension:
https://github.com/citusdata/citus/issues/2390

To reproduce:

CREATE OR REPLACE FUNCTION on_drop()
RETURNS event_trigger AS $ondrop$
BEGIN
  RAISE NOTICE 'drop_trigger';
END;
$ondrop$ LANGUAGE plpgsql;

CREATE EVENT TRIGGER fail_drop_table ON sql_drop
EXECUTE PROCEDURE on_drop();

CREATE TABLE collections_list (
  key bigint,
  ts timestamptz,
  collection_id integer,
  value numeric,
  PRIMARY KEY(collection_id)
) PARTITION BY LIST ( collection_id );

CREATE TABLE collections_list_1
PARTITION OF collections_list (key, ts, collection_id, value)
FOR VALUES IN (1);
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

Marco