Bug: WAIT FOR LSN crashes with assertion failure inside PL/pgSQL DO blocks and procedures

Hi hackers, Alexander,

An assertion failure (server crash in assert-enabled builds) occurs when
WAIT FOR LSN ... INTO is used inside PL/pgSQL DO blocks or within void
procedures.

Repro:

-- Run this on a standby

CREATE PROCEDURE test_wait()
LANGUAGE plpgsql AS $$
DECLARE
result text;
BEGIN
WAIT FOR LSN '0/1234' INTO result;
RAISE NOTICE '%', result;
END;
$$;
CALL test_wait();

The WAIT FOR itself succeeds, but the very next PL/pgSQL statement that
requires a snapshot crashes the backend with:

TRAP: failed Assert("portal->portalSnapshot == NULL"),
File: "pquery.c", Line: 1776

Attached patches for both the test case and a potential fix. Please review.

Thanks,
Satya