Fix undefined variable error.

lib/Cake/Controller/Component/SecurityComponent.php

@@ -553,12 +553,12 @@ protected function _validateCsrf($controller) {
  * @return array An array of nonce => expires.
  */
 	protected function _expireTokens($tokens) {
+		$now = time();
 		foreach ($tokens as $nonce => $expires) {
 			if ($expires < $now) {
 				unset($tokens[$nonce]);
 			}
 		}
-		$now = time();
 		$overflow = count($tokens) - $this->csrfLimit;
 		if ($overflow > 0) {
 			$tokens = array_slice($tokens, $overflow + 1, null, true);