Added signup table if missing

@@ -23,6 +23,11 @@ const ACTION_ACTIVATE = 'wpmu_activate_user';
 const OPTION_NAME = 'tz_auth'; // Database lookup key (`wp_options`.`option_name`)
 
     call_user_func(function() {
+        global $wpdb;
+        if (empty($wpdb->signups)) {
+            $wpdb->signups = $wpdb->prefix . 'signups';
+        }
+
         Vars::$options = new Tools\WP_Option(OPTION_NAME);
 
         if (is_admin()) {