Flushing ZlibEncoder loops infinitely on some writers

[HeroicKatora]

When the underlying writer only accepts very small write request the flush process of a ZlibEncoder appears to try and write the same data infinitely often.

Full reproduction source code

use std::io::{self, Write};

fn main() {
    let _ = deflate::write::ZlibEncoder::new(SmallWriter::new(vec![], 2), deflate::Compression::Fast).flush();
}

struct SmallWriter<W: Write> {
    writer: W,
    small: usize,
}

impl<W: Write> SmallWriter<W> {
    fn new(writer: W, buf_len: usize) -> SmallWriter<W> {
        SmallWriter {
            writer,
            small: buf_len,
        }
    }
}

impl<W: Write> Write for SmallWriter<W> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        // Never write more than `small` bytes at a time.
        let small = buf.len().min(self.small);
        self.writer.write(&buf[..small])
    }

    fn flush(&mut self) -> io::Result<()> {
        Ok(())
    }
}

[HeroicKatora]

Discovered in image-rs/image-png#206

[oyvindln]

I think it may be due to flush trying to output an empty block stored block (as per zlib sync flush) at the end when called, which won't fit in two bytes. Will try to find some way of solving it.