Feeds: Avoid notices in `get_post_comments_feed_link()`.

When an feed request is made to a non-existent page, surpress the notice.

Props dd32, SergeyBiryukov, mukesh27, hellofromTonya.
Fixes #52814.


Built from https://develop.svn.wordpress.org/trunk@51121


git-svn-id: http://core.svn.wordpress.org/trunk@50730 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/link-template.php

@@ -735,7 +735,7 @@ function get_feed_link( $feed = '' ) {
  * @param int    $post_id Optional. Post ID. Default is the ID of the global `$post`.
  * @param string $feed    Optional. Feed type. Possible values include 'rss2', 'atom'.
  *                        Default is the value of get_default_feed().
- * @return string The permalink for the comments feed for the given post.
+ * @return string The permalink for the comments feed for the given post on success, empty string on failure.
  */
 function get_post_comments_feed_link( $post_id = 0, $feed = '' ) {
 	$post_id = absint( $post_id );
@@ -749,6 +749,12 @@ function get_post_comments_feed_link( $post_id = 0, $feed = '' ) {
 	}
 
 	$post = get_post( $post_id );
+
+	// Bail out if the post does not exist.
+	if ( ! $post instanceof WP_Post ) {
+		return '';
+	}
+
 	$unattached = 'attachment' === $post->post_type && 0 === (int) $post->post_parent;
 
 	if ( get_option( 'permalink_structure' ) ) {

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-51120';
+$wp_version = '5.8-alpha-51121';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.