Handle the case if MigratePathautoHandler runs before MigratePathHandler - loss of alias data

In the case that we would do the following:

    $this->addFieldMapping('path', 'field')
      ->description(t('Setting manual alias'));

    $this->addFieldMapping('pathauto')
      ->description(t('Disabling pathauto'))
      ->defaultValue(0);

I was thinking possibly there might be a problem if MigratePathautoHandler runs before MigratePathHandler since the latter relies on $entity->path being set as the URL alias, but the former doesn't account for that. This may not be a real issue since I am uncertain of the actual order of operations for those handlers or if they could possibly run in the reverse order, but it seems a good thing to have fixed just in case, if not for showing that these types of conditions could exist and users will need to be cautious of them.