Fixed an issue in the destructor

...which would trigger a warning message if there were no queries

Zebra_Database.php

@@ -24,8 +24,8 @@
  *  For more resources visit {@link http://stefangabos.ro/}
  *
  *  @author     Stefan Gabos <[email protected]>
- *  @version    2.8.6 (last revision: December 28, 2014)
- *  @copyright  (c) 2006 - 2014 Stefan Gabos
+ *  @version    2.8.7 (last revision: February 10, 2015)
+ *  @copyright  (c) 2006 - 2015 Stefan Gabos
  *  @license    http://www.gnu.org/licenses/lgpl-3.0.txt GNU LESSER GENERAL PUBLIC LICENSE
  *  @package    Zebra_Database
  */
@@ -4415,7 +4415,7 @@ function __destruct()
     {
 
         // if the last result is a mysqli result set (it can also be a boolean or not set)
-        if ($this->last_result instanceof mysqli_result)
+        if (isset($this->last_result) && $this->last_result instanceof mysqli_result)
 
             // frees the memory associated with the last result
             mysqli_free_result($this->last_result);