Message 'Cannot send email about %subject without a recipient.' logged with bad param

htmlmail.mail.inc contains the following code:

watchdog(
          'HTMLMailSystem',
          'Cannot send email about %subject without a recipient.',
          array('subject' => $message['subject']),
          WATCHDOG_ERROR
        );

'subject' should be changed to '%subject' for a correct replacement.

This has already been fixed for Drupal 8.