REPACK (CONCURRENTLY) fails when replica identity index is dropped

I don't fully understand the mechanics of this one, but here is a
reproducer:

CREATE TABLE t (a INT PRIMARY KEY, b INT, c TEXT);
INSERT INTO t SELECT g, g, repeat('x', 1000) FROM generate_series(1, 1000000) g;
CREATE UNIQUE INDEX i ON t (a);
ALTER TABLE t REPLICA IDENTITY USING INDEX i;
DROP INDEX i;
REPACK (CONCURRENTLY) t;

-- in a separate session, while REPACK is still running
DELETE FROM t WHERE a = 1;

This produces the following ERROR from the REPACK command:

ERROR: incomplete delete info
CONTEXT: slot "pg_repack_34213", output plugin "pgrepack", in the change callback, associated LSN 0/A6CC1ED0
REPACK decoding worker

I think this can be addressed by verifying the index exists in
check_concurrent_repack_requirements() and erroring out if it doesn't.

--
nathan