Unchecking Override System Messages results in an empty $message var

If you uncheck the 'Override System Messages' option, jgrowl does not pass control back to the regular system message theme function.

I added a check against JGROWL_SYSTEM_MESSAGES before overriding the status_message theme function and that fixed the problem:

function jgrowl_theme_registry_alter(&$theme_registry) {
  if ((user_access('access jgrowl'))) {
    if (!empty($theme_registry['status_messages']) && JGROWL_SYSTEM_MESSAGES) {
      $theme_registry['status_messages']['function'] = 'jgrowl_system_messages';
    }
  }
}