Better handling of EOF in read_exact_bytes

[fintelia]

After e69b4c5 I'm seeing that trying to read from a BufReader with a small buffer no longer works (using the wrapper from image-rs/image#2198):

let decoder = JpegDecoder::new(BufReader::with_capacity(32, Cursor::new(include_bytes!("test.jpg")))).unwrap();
let mut decoded = vec![0; decoder.total_bytes() as usize];
decoder.read_image(&mut decoded).unwrap();

Produces:

I/O errors Not enough bytes, expected 14 but found 3144

The error is because BufReader sometimes returns fewer bytes than requested in a read call without it indicating that the end-of-input has been reached.

This PR changes the implementation of ZByteReaderTrait::read_exact_bytes to only treat zero bytes read as meaning EOF and all other partial reads get re-attempted

[etemesi254]

Yea this makes more sense, thank you.

Sorry for missing that, the docs are pretty weird on the behavior.