Fix #3529

Seems to be some really obscure PHP bug ...
diff --git a/system/libraries/Session/drivers/Session_database_driver.php b/system/libraries/Session/drivers/Session_database_driver.php
index 76596f0..0ec6e34 100644
--- a/system/libraries/Session/drivers/Session_database_driver.php
+++ b/system/libraries/Session/drivers/Session_database_driver.php
@@ -83,7 +83,7 @@
 
 		$CI =& get_instance();
 		isset($CI->db) OR $CI->load->database();
-		$this->_db =& $CI->db;
+		$this->_db = $CI->db;
 
 		if ( ! $this->_db instanceof CI_DB_query_builder)
 		{