message: decode_node_ack_or_syn(): Check buffer size
  Fixes
  thread 'tokio-runtime-worker-1' panicked at 'assertion failed: self.remaining() >= dst.len()', /home/eelco/.cargo/registry/src/github.com-1e
  cc6299db9ec823/bytes-0.4.10/src/buf/buf.rs:241:9
  when the incoming data block is smaller than an ACK/SYN
  message. (E.g. a "get tip" CBOR message is smaller.)
diff: @@ -178,6 +178,7 @@ impl Message {
  fn decode_node_ack_or_syn(lwcid: nt::LightWeightConnectionId, bytes: &Bytes) -> Option<Message> {
  use bytes::{Buf, IntoBuf};
  + if bytes.len() != 9 { return None }
  let mut buf = bytes.into_buf();
  let key = buf.get_u8();
  let v = buf.get_u64_be();
  