BUG #16856: Crash when add "_RETURN" rule on child table

The following bug has been logged on the website:

Bug reference:      16856
Logged by:          Yang Lin
Email address:      todoubaba@gmail.com
PostgreSQL version: 12.5
Operating system:   ubuntu 20.04
Description:

create table parent(a text);
create table child() inherits (parent);
create or replace rule "_RETURN" as
    on select
    to child
    do instead
    select *
    from (values('x')) as t(a);

select * from parent;

The psql crash and report:
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.