BUG #13571: sql-only extension segfaults backend during creation

The following bug has been logged on the website:

Bug reference:      13571
Logged by:          Feike Steenbergen
Email address:      feikesteenbergen@gmail.com
PostgreSQL version: 9.5alpha2
Operating system:   Debian jessie x86_64
Description:

When installing a sql-only extension, my backend crashes taking the cluster
with it.

After some debugging we have created a reproducable testcase.

postgres@postgres=# create extension castbug;
server closed the connection unexpectedly

castbug.control:

relocatable = false
superuser   = true
comment     = 'This triggers a segfault on 9.5alpha2'
default_version = unstable

castbug--unstable.sql:

DO
$$
DECLARE
    roles text[] := '{"job_scheduler","job_monitor"}';
BEGIN
END;
$$;

SELECT count(*)
  FROM pg_class
CROSS JOIN generate_series(1,100);

DO
$$
DECLARE
    something text[] := '{"jb_scheduler","job_monitr"}';
BEGIN
END;
$$;