Fix a wrong variable name from previous commit

Rel: #3186
diff --git a/system/libraries/Cache/drivers/Cache_redis.php b/system/libraries/Cache/drivers/Cache_redis.php
index ab4fd0d..7c9da3d 100644
--- a/system/libraries/Cache/drivers/Cache_redis.php
+++ b/system/libraries/Cache/drivers/Cache_redis.php
@@ -300,7 +300,7 @@
 		$serialized = $this->_redis->sMembers('_ci_redis_serialized');
 		if ( ! empty($serialized))
 		{
-			$this->_serialized = array_flip($this->_serialized);
+			$this->_serialized = array_flip($serialized);
 		}
 
 		return TRUE;