common: disallow NULL channel_id to peer_failed_err.

No more sending "all-channel" errors; in particular, gossipd now only
sends warnings (which make us hang up), not errors, and peer_connected
rejections are warnings (and disconnect), not errors.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: Plugins: `peer_connected` rejections now send a warning, not an error, to the peer.

common/peer_failed.c

@@ -71,6 +71,7 @@ void peer_failed_err(struct per_peer_state *pps,
 	va_list ap;
 	const char *desc;
 
+	assert(channel_id);
  	va_start(ap, fmt);
 	desc = tal_vfmt(tmpctx, fmt, ap);
 	va_end(ap);