Check data with isset() before accessing it.

Don't blindly access data that may not exist.

Fixes #2712

lib/Cake/Model/Model.php

@@ -2026,7 +2026,9 @@ class Model extends Object implements CakeEventListener {
  * @return void
  */
 	public function updateCounterCache($keys = array(), $created = false) {
-		$keys = empty($keys) ? $this->data[$this->alias] : $keys;
+		if (empty($keys) && isset($this->data[$this->alias])) {
+			$keys = $this->data[$this->alias];
+		}
 		$keys['old'] = isset($keys['old']) ? $keys['old'] : array();
 
 		foreach ($this->belongsTo as $parent => $assoc) {