guard against undefined notices when getting custom post permalinks

have seen this before, but didn't isolate/troubleshoot it to completion. suspect within URL detection.

may be possible to replicate by installing/removing WooCommerce and some products or some custom post types

notice logged when occurring:

```
Notice: Trying to get property 'hierarchical' of non-object in .../wp-includes/link-template.php on line 282
Notice: Trying to get property 'query_var' of non-object in .../wp-includes/link-template.php on line 292
```

- reported by user at https://staticword.press/t/crawling-not-working
- https://wpml.org/forums/topic/notice-trying-to-get-property-of-non-object-in-link-template-php-on-line-49/

suggest debugging the `link-template.php` and see which links it's failing on.