Decoding doesn't always return `None` when it should.

[JayWhite2357]

[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x02] is the encoding of u64::MAX.
This does not return None when decoding even though it should.

I have implemented a fix here: #36.