routing: Do not set an empty channel_announcement if none is given

Signed-off-by: Christian Decker <decker.christian@gmail.com>

gossipd/routing.c

@@ -424,9 +424,11 @@ static bool add_channel_direction(struct routing_state *rstate,
 	c = half_add_connection(rstate, from, to, short_channel_id, direction);
 
 	/* Remember the announcement so we can forward it to new peers */
+	if (announcement) {
 		tal_free(c->channel_announcement);
 		c->channel_announcement = tal_dup_arr(c, u8, announcement,
 						      tal_count(announcement), 0);
+	}
 	return true;
 }