message: [dns] fix potential discovery proxy crash
  This commit fixes potential Discovery Proxy crash when the mDNS
  implementation replied with an empty host name.
diff: @@ -223,11 +223,17 @@ void DiscoveryProxy::OnHostDiscovered(const std::string &
  {
  otDnssdHostInfo hostInfo;
  const otDnssdQuery *query = nullptr;
  + std::string resolvedHostName = aHostInfo.mHostName;
  otbrLogInfo("host discovered: %s hostname %s addresses %zu", aHostName.c_str(), aHostInfo.mHostName.c_str(),
  aHostInfo.mAddresses.size());
  - CheckHostnameSanity(aHostInfo.mHostName);
  + if (resolvedHostName.empty())
  + {
  + resolvedHostName = aHostName + ".local.";
  + }
  +
  + CheckHostnameSanity(resolvedHostName);
  hostInfo.mAddressNum = aHostInfo.mAddresses.size();
  if (!aHostInfo.mAddresses.empty())
  @@ -257,7 +263,7 @@ void DiscoveryProxy::OnHostDiscovered(const std::string &
  if (hostName == aHostName)
  {
  - std::string hostFullName = TranslateDomain(aHostInfo.mHostName, domain);
  + std::string hostFullName = TranslateDomain(resolvedHostName, domain);
  otDnssdQueryHandleDiscoveredHost(mNcp.GetInstance(), hostFullName.c_str(), &hostInfo);
  }
  