Writing 0 bytes to a full buffer results in an error

[arjantop]

use std::io::BufWriter;

fn main() {
    let mut buf = [0u8, ..5];
    let mut writer = BufWriter::new(buf);
    writer.write([1, 2, 3, 4, 5]).unwrap();
    writer.write([]).unwrap();
}

http://is.gd/1lwQuf