message: [agent] fix the process of generating extPanId
diff: @@ -210,7 +210,9 @@ void ThreadHelper::Attach(const std::string & aNetworkName,
  }
  else
  {
  - while (aExtPanId != UINT64_MAX)
  + *reinterpret_cast<uint64_t *>(&extPanId) = UINT64_MAX;
  +
  + while (*reinterpret_cast<uint64_t *>(&extPanId) == UINT64_MAX)
  {
  RandomFill(extPanId.m8, sizeof(extPanId.m8));
  }
  