lightningd: tell gossipd channel is closed if it tells us about our channel and is wrong.

While we have (I hope!) fixed the underlying sync problem, this can still happen with
older gossip.  So now we tell it the channel is dead, so it won't happen more than once.

Fixes: #7703
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/gossip_control.c

@@ -134,8 +134,13 @@ static void handle_init_cupdate(struct lightningd *ld, const u8 *msg)
 
 	channel = any_channel_by_scid(ld, scid, true);
 	if (!channel) {
-		log_broken(ld->log, "init_cupdate for unknown scid %s",
+		log_broken(ld->log, "init_cupdate for unknown scid %s: telling gossipd it's spent",
 			   fmt_short_channel_id(tmpctx, scid));
+		/* Presumably gossipd missed that it was already spent, so
+		 * tell it now! (We need a tal object for this, hence
+		 * tal_dup) */
+		gossipd_notify_spends(ld, ld->gossip_blockheight,
+				      tal_dup(tmpctx, struct short_channel_id, &scid));
 		return;
 	}