Avoid integer overflow when parsing frame bytes.

[frewsxcv]

The following program causes a panic, prior to these changes.

extern crate ws;

fn main() {
    let bytes = b"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xfe\xfe\xfe\xfe\xfe\t\x01\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xfe\xb1\n".to_vec();
    let mut data = std::io::Cursor::new(bytes);
    ws::Frame::parse(&mut data);
}

Found via cargo-fuzz.