message: Do not flush when there is nothing to write
diff: @@ -1235,9 +1235,11 @@ inline File::Streambuf::pos_type File::Streambuf::seekpos(pos_type pos, std::ios
  inline void File::Streambuf::flush()
  {
  size_t n = pptr() - pbase();
  + if (n > 0) {
  m_file.write(pbase(), n);
  setp(m_buffer.get(), epptr());
  }
  +}
  inline File::AccessError::AccessError(const std::string& msg, const std::string& path)
  : std::runtime_error(msg)
  