check if a $_SESSION entry exists before we try to remove it

lib/private/session/internal.php

@@ -35,7 +35,9 @@ class Internal extends Memory {
 	 */
 	public function remove($key) {
 		// also remove it from $_SESSION to prevent re-setting the old value during the merge
-		unset($_SESSION[$key]);
+		if (isset($_SESSION[$key])) {
+			unset($_SESSION[$key]);
+		}
 		parent::remove($key);
 	}