Fix: normalize $post_id and skip id 0 in post edit/change cache hooks (#1070)

wp_cache_post_edit() and wp_cache_post_change() assumed $post_id was
always an integer. Defensively normalize a WP_Post object down to its
ID, cast to int, and bail out early when the id is 0 so a stray
zero-id invalidation can't wipe the front-page/root cache.

Refs #959

Author: donnchawp

wp-cache-phase2.php

@@ -3287,6 +3287,16 @@ function wp_cache_post_edit( $post_id ) {
 	global $wp_cache_clear_on_post_edit, $cache_path, $blog_cache_dir;
 	static $last_post_edited = -1;
 
+	// Normalize $post_id to always be an integer (handles both int and WP_Post object)
+	if ( is_object( $post_id ) && isset( $post_id->ID ) ) {
+		$post_id = $post_id->ID;
+	}
+	$post_id = (int) $post_id;
+
+	if ( $post_id === 0 ) {
+		return $post_id;
+	}
+
 	if ( $post_id == $last_post_edited ) {
 		$action = current_filter();
 		wp_cache_debug( "wp_cache_post_edit({$action}): Already processed post $post_id.", 4 );
@@ -3360,6 +3370,16 @@ function wp_cache_post_change( $post_id ) {
 	global $file_prefix, $cache_path, $blog_id, $super_cache_enabled, $blog_cache_dir, $wp_cache_refresh_single_only;
 	static $last_processed = -1;
 
+	// Normalize $post_id to always be an integer (handles both int and WP_Post object)
+	if ( is_object( $post_id ) && isset( $post_id->ID ) ) {
+		$post_id = $post_id->ID;
+	}
+	$post_id = (int) $post_id;
+
+	if ( $post_id === 0 ) {
+		return $post_id;
+	}
+
 	if ( $post_id == $last_processed ) {
 		$action = current_filter();
 		wp_cache_debug( "wp_cache_post_change({$action}): Already processed post $post_id.", 4 );