Handle zero-length bundles

[jwt27]

Receiving empty bundles doesn't seem to work - the element size is parsed, but never used, and it ends up throwing an exception in the Message address parser.

A quick fix could be:

--- a/src/bundle.js
+++ b/src/bundle.js
@@ -150,6 +150,9 @@ export default class Bundle {

       offset = size.unpack(dataView, offset)

+      if (size.value == 0)
+        continue;
+
       // check if Packet is a Bundle or a Message
       let item
       head.unpack(dataView, offset)

Although I think a more "correct" approach would involve using a new DataView for each bundle element.