message: [mdns] fix crash upon avahi access error
diff: @@ -511,6 +511,7 @@ otbrError PublisherAvahi::PublishService(uint16_t aPort, const char *aName, cons
  va_start(args, aType);
  VerifyOrExit(mState == kStateReady, errno = EAGAIN);
  + VerifyOrExit(mGroup != nullptr, ret = OTBR_ERROR_MDNS);
  for (const char *name = va_arg(args, const char *); name; name = va_arg(args, const char *))
  {
  