In `_wp_json_convert_string()`, when `$use_mb` is `false`, perhaps pass a variable that actually exists to `wp_check_invalid_utf8()`.

Introduced in [30055].
See #30224.


git-svn-id: https://develop.svn.wordpress.org/trunk@30162 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -2739,7 +2739,7 @@ function _wp_json_convert_string( $string ) {
 			return mb_convert_encoding( $string, 'UTF-8', 'UTF-8' );
 		}
 	} else {
-		return wp_check_invalid_utf8( $data, true );
+		return wp_check_invalid_utf8( $string, true );
 	}
 }