message: Add thread safety annotations to RNG state
diff: @@ -297,10 +297,11 @@ void GetRandBytes(unsigned char* buf, int num)
  namespace {
  struct RNGState {
  Mutex m_mutex;
  - unsigned char m_state[32] = {0};
  - uint64_t m_counter = 0;
  + unsigned char m_state[32] GUARDED_BY(m_mutex) = {0};
  + uint64_t m_counter GUARDED_BY(m_mutex) = 0;
  - explicit RNGState() {
  + RNGState()
  + {
  InitHardwareRand();
  }
  };
  