Infinite loop writing an empty buffer in write::BzDecoder

[naufraghi]

This minimal example is never returning with the current master (3032f37).

extern crate bzip2;

use bzip2::write::BzDecoder;
use std::io::prelude::*;

fn main() {
    let mut d = BzDecoder::new(Vec::new());
    d.write(b"").unwrap();
    d.finish().unwrap();
}

With a similar test case cargo test was stuck:

    #[test]
    fn write_empty() {
        let d = BzDecoder::new(Vec::new());
        let mut c = BzEncoder::new(d, ::Compression::default());
        c.write(b"").unwrap();
        let data = c.finish().unwrap().finish().unwrap();
        assert_eq!(&data[..], b"");
    }

Diving into the simpler example traceback the problem seems related to try_finish in write.rs, where we are writing an empty buffer forever:

    pub fn try_finish(&mut self) -> io::Result<()> {
        while !self.done {
            self.write(&[])?;
        }
        self.dump()
    }

Maybe it is sufficient to check if we received some input? Like:

    pub fn try_finish(&mut self) -> io::Result<()> {
        // If nothing entered, there is no need to loop
        while !self.done && self.total_in() != 0 {
            self.write(&[])?;
        }
        self.dump()
    }

That done cargo test works with the added test too.

Let me know if you'd like a Pull Request, it's ready on my disk.