Fix bug in add nested group (UserStorageDB)

SE/se-lib/UserStorageDB.php

@@ -256,7 +256,7 @@ class UserStorageDB extends UserStorageBase {
 	public function addNestedGroup($groupID, $nestedGroupID) {
 		if (!$this->_db) return null;
 		if ($groupID <= 0) return null;
-		if ($nestedGroupID) return null;
+		if ($nestedGroupID <= 0) return null;
 		return $this->_setGroupConnection($groupID, $nestedGroupID);
 	}