wp_get_shortlink produces PHP Notice on unregistered post type

[apedog]

Calling wp_get_shortlink with an existing ID of an unregistered custom post type, the following PHP notice is shown:
Notice: Trying to get property 'public' of non-object in \wp-includes\link-template.php on line 3913

Step to reproduce:

1. Create a custom post type.
2. Add new post of custom post type to database.
3. Unregister/disable custom post type.
4. Call wp_get_shortlink with ID from step 2

Problem is due to get_post( $id ) returning object regardless if registered or not. So $post_id is not empty.
However get_post_type_object returns null with no sanity checks before $post_type->public is tested.

[apedog]

51376 - alternative patch

[SergeyBiryukov]

In 49067:

Posts, Post Types: Avoid a PHP notice in wp_get_shortlink() if the post type is no longer registered.

Props apedog.
Fixes #51376.