fix: BytesMut only reuse if src has remaining

[discord9]

prevent stack overflow in this case:

let mut buf = BytesMut::new();
buf.put(&[] as &[u8]);

closes #802

[discord9]

@paolobarbolini is this fix reasonable?

[Darksonn]

Thanks for finding this!

Overall looks good, but I have a suggested alternative implementation.

[Darksonn]

Thanks.