message: [mdns] fix deallocation of null pointers
diff: @@ -243,8 +243,11 @@ void PublisherMDnsSd::Stop(void)
  mServices.clear();
  otbrLog(OTBR_LOG_INFO, "[mdns] remove all hosts");
  + if (mHostsRef != nullptr)
  + {
  DNSServiceRefDeallocate(mHostsRef);
  mHostsRef = nullptr;
  + }
  mHosts.clear();
  exit:
  