message: truncate reason string
diff: // The class implementing a QUIC connection.
  use std::cell::RefCell;
  -use std::cmp::max;
  +use std::cmp::{max, min};
  use std::collections::HashMap;
  use std::convert::TryFrom;
  use std::fmt::{self, Debug};
  @@ -1531,7 +1531,8 @@ impl Connection {
  if let CloseError::Transport(_) = error_code {
  builder.encode_varint(*frame_type);
  }
  - builder.encode_vvec(reason_phrase);
  + let reason_len = min(min(reason_phrase.len(), 256), builder.remaining());
  + builder.encode_vvec(&reason_phrase[..reason_len]);
  } else {
  unreachable!();
  }
  