BUG #15327: postgres segfaults on ALTER FUNCTION ... SET SCHEMA ...

The following bug has been logged on the website:

Bug reference:      15327
Logged by:          Kai Kratz
Email address:      kai.kratz@googlemail.com
PostgreSQL version: 10.4
Operating system:   Ubunt18.04
Description:

I observed a 100% repeatable crash when doing the following:
pg-user=# CREATE SCHEMA test_schema;
CREATE SCHEMA
pg-user=# CREATE FUNCTION custom_eq(int, int)
RETURNS BOOL
LANGUAGE SQL IMMUTABLE AS
$$
SELECT $1 = $2;
$$;
CREATE FUNCTION
pg-user=# ALTER FUNCTION custom_eq(int, int) SET SCHEMA test_schema;
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.
!>