session_cache_limiter() goes before session_start()

Fixes issue https://github.com/simplesamlphp/simplesamlphp/issues/793

lib/SimpleSAML/SessionHandlerPHP.php

@@ -112,8 +112,8 @@ class SessionHandlerPHP extends SessionHandler
              */
             session_cache_limiter('');
         }
-        @session_start();
         session_cache_limiter($cacheLimiter);
+        @session_start();
     }