I know that this is not the only tutorial that has code that check nonce for meta boxes but this is truly idiotic. Nonce should be checked per the whole action not per parts of it and if your save_post hook was called it means that the save nonce was already checked and found valid, so There is no need for more security checks.
My advice is to simply remove anything related to nonce from your metabox code, but if you will feel better having it around just change the code to
// verify nonce
if ( !isset($_POST[ 'my_meta_box_nonce' ]) || !wp_verify_nonce( $_POST[ 'my_meta_box_nonce' ], basename( __FILE__ ) ) )
return $post_id;
which will bail out when no nonce was supplied or the nonce failed validation