Fix a number of CI_Cache bugs

Fixes #4277
Supersedes #4474
Really fixes #4066
diff --git a/system/libraries/Cache/drivers/Cache_memcached.php b/system/libraries/Cache/drivers/Cache_memcached.php
index c44958b..7e67fc4 100644
--- a/system/libraries/Cache/drivers/Cache_memcached.php
+++ b/system/libraries/Cache/drivers/Cache_memcached.php
@@ -107,6 +107,7 @@
 		else
 		{
 			log_message('error', 'Cache: Failed to create Memcache(d) object; extension not loaded?');
+			return;
 		}
 
 		foreach ($this->_memcache_conf as $cache_server)