Non-standard URL schemes with more than just hostname fail validation.

This is another but introduced by #2247261: URLs are not validated the 7.x-1.4 release.
I.e. this is another sibling of #2666912: URL validation rejects existing valid content after upgrade to 7.x-1.4. But that issue does not have a clear problem description, so I'm making a new issue.

Problem/Motivation

drupal_valid_path() has no good support for non-standard URL schemes; e.g.
irc://host passes
irc://host/path fails.

This is because the following code block is executed rather arbitrarily:

    $flag = valid_url($text, $type == LINK_EXTERNAL);
    if (!$flag) {
      $normal_path = drupal_get_normal_path($text, $langcode);
      $parsed_link = parse_url($normal_path, PHP_URL_PATH);
      if ($normal_path != $parsed_link) {
        $normal_path = $parsed_link;
      }
      $flag = drupal_valid_path($normal_path);
    }

In the first case, parse_url($normal_path, PHP_URL_PATH); is NULL, and drupal_valid_path(NULL) is TRUE... in most cases.
In the second case, parse_url($normal_path, PHP_URL_PATH); is a string, and drupal_valid_path(NULL) is FALSE... in most cases.
But really this whole logic should not be executed for these non-standard scheme.

Steps to reproduce

See test fail.

Proposed resolution

See patch.