Update typecho_redis.class.php

close #5

Author: phpgao

driver/typecho_redis.class.php

@@ -16,7 +16,7 @@ private function __construct($option=null) {
     }
 
     static public function getInstance($option) {
-        if (is_null ( self::$_instance ) || isset ( self::$_instance )) {
+        if (is_null ( self::$_instance ) || ! isset ( self::$_instance )) {
             self::$_instance = new self($option);
         }
         return self::$_instance;
@@ -56,4 +56,4 @@ public function flush()
     {
         return $this->redis->flushDB();
     }
-}
+}