blockAccess needs to return AccessResult

Currently the d8 block is crashing the site.
\Drupal\masquerade\Plugin\Block\MasqueradeBlock::blockAccess should read:

  protected function blockAccess(AccountInterface $account) {
    if ($account->hasPermission('masquerade as any user') && !$this->masquerade->isMasquerading()) {
      return AccessResult::allowed()
    }
    return AccessResult::forbidden();
  }