Use $_GET[destination] in preference to $_SESSION to avoid strange success destination redirect behaviour

On the - admittedly strangely-built - site I'm currently working on, the Agreement module's use of $_SESSION to store the last-requested URL is resulting in users being sent back to paths such as sites/all/themes/foo/image.gif.

This is probably a result of the theme having one or two missing images which are called later than the page itself.

The simplest solution here would probably be to use $_GET[destination] in preference to $_SESSION; the code changes below prioritise $_GET over $_SESSION, but will fall back to the original module behaviour if something isn't working properly.

Change:

        drupal_goto(check_plain(variable_get('agreement_page_url',
        AGREEMENT_PAGE_URL)));
        exit();

... to:

        drupal_goto(check_plain(variable_get('agreement_page_url', AGREEMENT_PAGE_URL)), drupal_get_destination());
        exit();

and:

  elseif ($agreement_success_destination == '') {
    if ($_SESSION['agreement_destination'] == '') {
      $redirect = '<front>';
    }
    else {
      $redirect = $_SESSION['agreement_destination'];
    }
  }

... to:

  elseif ($agreement_success_destination == '') {
    if (isset($_GET['destination'])) {
      $redirect = $_GET['destination'];
    }
    elseif ($_SESSION['agreement_destination'] == '') {
      $redirect = '<front>';
    }
    else {
      $redirect = $_SESSION['agreement_destination'];
    }
  }

Again, apologies for the lack of a nice, sensible diff file :-$