[PATCH] Fix leaked hash scans in pg_listening_channels()

Hi hackers,

I found a hash scan leak in pg_listening_channels(), introduced by
282b1cde9d.

LISTEN ch;
SELECT pg_listening_channels() LIMIT 1;

The SELECT emits a leak warning in the server logs.

Repeated early exits can exhaust the hash scan limit and cause a
server crash.

Attached patch uses InitMaterializedSRF() to complete the scan
before returning rows, following 257c8231bf.

/Joel