message: [coap] fix write outside buffer bounds
  Lack of (unsigned char *) cast a pointer to structure pdu->hdr is
  causing writes outside CoAP PDU buffer bounds.
diff: @@ -181,7 +181,7 @@ otbrError AgentLibcoap::Send(Message & aMessage,
  VerifyOrExit(pdu->length + sizeof(meta) < pdu->max_size, errno = EMSGSIZE);
  tid = coap_send_confirmed(&mCoap, mCoap.endpoint, &remote, pdu);
  - memcpy(pdu->hdr + pdu->length, &meta, sizeof(meta));
  + memcpy((unsigned char *)pdu->hdr + pdu->length, &meta, sizeof(meta));
  }
  else
  {
  @@ -269,7 +269,7 @@ void AgentLibcoap::HandleResponse(coap_context_t * aCoap,
  VerifyOrExit(aSent != NULL, otbrLog(OTBR_LOG_ERR, "request not found!"));
  - memcpy(&meta, aSent->hdr + aSent->length, sizeof(meta));
  + memcpy(&meta, (unsigned char *)aSent->hdr + aSent->length, sizeof(meta));
  if (meta.mHandler)
  {
  