Customize: Safeguard a check on the `customize_validate_{$setting_id}` filter value to ensure it is a `WP_Error`.

While the filter is documented to only support a `WP_Error`, it has been a common practice to return true in a validation function if no errors have occurred. This was already caught when the same filter was executed in `WP_Customize_Setting`, it was however missing in `WP_Customize_Manager::validate_setting_values()`.

Props flixos90.
Merges [43578] to the 4.9 branch.
Fixes #44809.
Built from https://develop.svn.wordpress.org/branches/4.9@43619


git-svn-id: http://core.svn.wordpress.org/branches/4.9@43448 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-manager.php

@@ -2284,7 +2284,7 @@ final class WP_Customize_Manager {
 			if ( ! is_wp_error( $validity ) ) {
 				/** This filter is documented in wp-includes/class-wp-customize-setting.php */
 				$late_validity = apply_filters( "customize_validate_{$setting->id}", new WP_Error(), $unsanitized_value, $setting );
-				if ( ! empty( $late_validity->errors ) ) {
+				if ( is_wp_error( $late_validity ) && ! empty( $late_validity->errors ) ) {
 					$validity = $late_validity;
 				}
 			}

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9.9-alpha-43618';
+$wp_version = '4.9.9-alpha-43619';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.