Body length 0 causes incorrect sig for validate

[wingedpig]

When verifying a signature, the code treats a body length of 0 as the same as no body length specified at all. What should actually happen is that if there is a body length of 0, then the message body is not signed and you generate a signature over no data. If there's no body length specified, then the code should sign over the entire message (this does happen now).