routing: Refuse self-route

Fixes #50. Was causing a segfault because it was creating an empty route
and was trying to extract the first hop as next hop. Routes through self
can still be created manually, but `sendpay` would still refuse to act
on them due to the missing self-link.

daemon/routing.c

@@ -289,10 +289,15 @@ struct peer *find_route(struct lightningd_state *dstate,
 	 * at the end, and need to derive how much we need to send. */
 	dst = get_node(dstate, &dstate->id);
 	src = get_node(dstate, to);
+
 	if (!src) {
 		log_info_struct(dstate->base_log, "find_route: cannot find %s",
 				struct pubkey, to);
 		return NULL;
+	} else if (dst == src) {
+		log_info_struct(dstate->base_log, "find_route: this is %s, refusing to create empty route",
+				struct pubkey, to);
+		return NULL;
 	}
 
 	/* Reset all the information. */