Some cleanup related to mt_rand()

- min and max values are 0 and mt_getrandmax() by default
- remove useless mt_srand() seed calls
diff --git a/system/core/Security.php b/system/core/Security.php
index 7aae54e..196d611 100644
--- a/system/core/Security.php
+++ b/system/core/Security.php
@@ -488,8 +488,7 @@
 	{
 		if ($this->_xss_hash === '')
 		{
-			mt_srand();
-			$this->_xss_hash = md5(time() + mt_rand(0, 1999999999));
+			$this->_xss_hash = md5(uniqid(mt_rand()));
 		}
 
 		return $this->_xss_hash;