[stubs] Properly handle length overflow in StringAddStub.

test/mjsunit/regress/regress-crbug-627934.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --stack-size=100
-
-(function rec() {
+var x = "1".repeat(32 * 1024 * 1024);
+for (var z = x;;) {
   try {
-    rec();
+    z += {toString: function() { return x; }};
   } catch (e) {
-    /{/;
+    break;
   }
-})();
+}