Fingerprint values sometimes aren't set due to negative result from bitwise xor

[andrewmbenton]

Due to the way JavaScript handles bitwise operands, I think the result of xor can be "negative".

This is causing the addFingerprint method to fail intermittently since the value is expected to be a positive integer. Here's the problematic line: https://github.com/reklatsmasters/stun/blob/cb84b7361d651742041f72f1ea247eb808c499e7/lib/message.js#L159

[reklatsmasters]

@andrewmbenton Thanks!