Panic in `bug` when parsing malformed file

```rust
#[test]
fn test_bug() {
    let input = b"\x04\x04\x04\x05:\x1az*\xfc\x06\x01\x90\x01\x06\x01";
    let mut decoder = Decoder::new(&input[..]);
                                                                        
    let result = io::copy(&mut decoder, &mut io::sink());
}
```

```rust
thread 'deflate::decode::tests::test_bug' panicked at 'bug', src/huffman.rs:124:43
```