multiple write! on async_std::fs::File can trigger infinite process

[hn3000]

By writing two short strings with separate write! invocations we can trigger an infinite process
that will continue writing the start of the second string until the process is killed or the disk is full.

    println!("starting ...");

    let mut file = OpenOptions::new()
                .append(true)
                // .write(true)    // also happens with write, write+truncate
                // .truncate(true)
                .create(true)
                .open(filename).await?;

    write!(
        &mut file, 
        "{}", 
        a, 
    ).await?;
    write!(
        &mut file, 
        "{}", 
        b, 
    ).await?;
    
    println!("done.");

As long as the second string b is shorter than the first one (a) we can't reproduce the problem, with a longer than 8 bytes we can reliably reproduce the problem.

Full reproduction in this gist: https://gist.github.com/hn3000/100460d1e239f22bf3862f1a8f8cfd92

(Also, I am not sure how to write a failing unit test because the error case does not terminate.)

[hn3000]

That PR definitely fixes it.