Assert failure when validating foreign keys

This results in an Assert failure on master and an elog ERROR prior to
c2fe139c201:

create role test_role with login;
create table ref(a int primary key);
grant references on ref to test_role;
set role test_role;
create table t1(a int, b int);
insert into t1 values(1,1);
alter table t1 add constraint t1_b_key foreign key (b) references ref(a);
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.

Fails in heapam_tuple_satisfies_snapshot() at
Assert(BufferIsValid(bslot->buffer));

c2fe139c201~1:
ERROR:  expected buffer tuple

The test case is just a variation of the case in [1], but a different
bug, so reporting it on a different thread.

I've not looked into the cause or when it started happening.

[1] https://www.postgresql.org/message-id/CAK%3D1%3DWrnNmBbe5D9sm3t0a6dnAq3cdbF1vXY816j1wsMqzC8bw%40mail.gmail.com

-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services