check for existence of node before checking it in let scoping

lib/6to5/transformation/transformers/let-scoping.js

@@ -260,7 +260,7 @@ LetScoping.prototype.checkFor = function () {
       return false;
     }
 
-    if (forParent && !node.label) {
+    if (forParent && node && !node.label) {
       if (t.isBreakStatement(node)) {
         has.hasBreak = true;
         replace = t.returnStatement(t.literal("break"));