verify exists index 0 in $ref

exemple

i have to get the referrer url from any page if exists

```
class AppController extends Controller {
......
    public function beforeRender() {
        parent::beforeRender();
        $this->params['referer'] = $this->referer(null, true);
    }
..........
}
```

lib/Cake/Network/CakeRequest.php

@@ -448,7 +448,7 @@ public function referer($local = false) {
 		if (!empty($ref) && !empty($base)) {
 			if ($local && strpos($ref, $base) === 0) {
 				$ref = substr($ref, strlen($base));
-				if ($ref[0] !== '/') {
+				if (isset($ref[0]) && $ref[0] !== '/') {
 					$ref = '/' . $ref;
 				}
 				return $ref;