[CA-148776] avoid sign extension breaking checksum algorithm

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
[CA-148776] Set correct offset to payload for packets which have an offset

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
Merge pull request #41 from benchalmers/ca-148776

Resolve checksum problems with UDP packets