AsyncLogging will crash if rotate() fails

AsyncLogWriter::write() invokes LogFileOutput::write_blocking() directly.

int LogFileOutput::write_blocking(const LogDecorations& decorations, const char* msg) {
  _rotation_semaphore.wait();
  int written = LogFileStreamOutput::write(decorations, msg);
  if (written > 0) {
    _current_size += written;

    if (should_rotate()) {
      rotate();
    }
  }
  _rotation_semaphore.signal();

  return written;
}

However, it doesn't put an exception case into consideraton.
rotate() may fail before and leave _stream NULL.

LogFileStreamOutput::write() will crash at flockfile(NULL)