fixup! sphinx: add check for blinding point

Author: gijswijs

sphinx.go

@@ -629,10 +629,11 @@ func (r *Router) ProcessOnionPacket(onionPkt *OnionPacket, assocData []byte,
 		o(cfg)
 	}
 
-	// If this is an onion message, a blinding point must be provided.
-	if cfg.isOnionMessage && cfg.blindingPoint == nil {
+	// If this is an onion message, a blinding point must be provided and
+	// associated data must be nil.
+	if cfg.isOnionMessage && cfg.blindingPoint == nil && assocData != nil {
 		return nil, fmt.Errorf("blinding point must be provided for " +
-			"onion messages")
+			"onion messages, and associated data must be nil")
 	}
 
 	// Compute the shared secret for this onion packet.