fix corner case in rename (#3481)

fixes #3480

lib/mozilla-ast.js

@@ -403,7 +403,7 @@
         var def = M.definition();
         return {
             type: "Identifier",
-            name: def ? def.mangled_name || def.name : M.name
+            name: def && def.mangled_name || M.name
         };
     });