gossipd: fix premature towire_reply_short_channel_ids_end if no node_announcement.

Our "are we finished?" logic was wrong: it tested if there are no more
node_announcements, but it's possible that there were no node_announcements
for either end of the channel whose information we sent.

This is actually quite unusual on the real network: looking at mainnet
statis from last May, 4301 of 4337 nodes have node_announcements.

However, with query flags it's much more likely, since they might not
ask for node announcements at all.

(Found by gossip protocol tests)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

gossipd/gossipd.c

@@ -1576,7 +1576,9 @@ static void maybe_create_next_scid_reply(struct peer *peer)
 	peer->scid_query_nodes_idx = i;
 
 	/* All finished? */
-	if (peer->scid_queries && peer->scid_query_nodes_idx == num) {
+	if (peer->scid_queries
+	    && peer->scid_query_idx == tal_count(peer->scid_queries)
+	    && peer->scid_query_nodes_idx == num) {
 		/* BOLT #7:
 		 *
 		 * - MUST follow these responses with