HPACK decoder panics on invalid input

Summary

Due to insufficient checking of input data, decoding certain data sequences can lead to _Decoder::decode_ panicking rather than returning an error. Example code that triggers this vulnerability looks like this: ```rust use hpack::Decoder; pub fn main() { let input = &[0x3f]; let mut decoder = Decoder::new(); let _ = decoder.decode(input); } ``` hpack is unmaintained. A crate with the panics fixed has been published as [hpack-patched](https://crates.io/crates/hpack-patched). Also consider using [fluke-hpack](https://crates.io/crates/fluke-hpack) or [httlib-huffman](https://crates.io/crates/httlib-huffman) as an alternative.