Handle transfer with no payload

Author: grs

lib/session.js

@@ -312,7 +312,11 @@ Incoming.prototype.on_transfer = function(frame, receiver) {
             if (util.is_defined(frame.performative.delivery_id) && current.id !== frame.performative.delivery_id) {
                 throw Error('frame sequence error: delivery ' + current.id + ' not complete, got ' + frame.performative.delivery_id);
             }
-            data = Buffer.concat([current.data, frame.payload], current.data.length + frame.payload.length);
+            if (frame.payload) {
+                data = Buffer.concat([current.data, frame.payload], current.data.length + frame.payload.length);
+            } else {
+                data = current.data;
+            }
         } else if (this.next_delivery_id === frame.performative.delivery_id) {
             current = {'id':frame.performative.delivery_id,
                 'tag':frame.performative.delivery_tag,