[ci skip] Merge pull request #5379 from tianhe1986/develop_session_memcached

Fix of Session_memcached_driver::_get_lock 
diff --git a/system/libraries/Session/drivers/Session_memcached_driver.php b/system/libraries/Session/drivers/Session_memcached_driver.php
index edabff5..b109738 100644
--- a/system/libraries/Session/drivers/Session_memcached_driver.php
+++ b/system/libraries/Session/drivers/Session_memcached_driver.php
@@ -313,6 +313,8 @@
 					? $this->_memcached->add($this->_lock_key, time(), 300)
 					: FALSE;
 			}
+
+			return TRUE;
 		}
 
 		// 30 attempts to obtain a lock, in case another request already has it