fix a bug while use database prefix other than "shj_"

i use a different database_prefix and it lead to the error below

A Database Error Occurred

Error Number: 1146

Table 'sdoj.sdoj_shj_sessions' doesn't exist

SELECT * FROM `sdoj_shj_sessions` WHERE `session_id` = '14810b1e40d1a6485647ae991f8e362b' AND `user_agent` = 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.114 Safari/537.36' LIMIT 1

Filename: libraries/Session/drivers/Session_cookie.php

Line Number: 454