Fix bug in keyed load stub for strings.

Instead of returning the empty string when indexing
a string out of bounds we now correctly return undefined.

Review URL: http://codereview.chromium.org/542089

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3611 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/runtime.cc

@@ -1479,7 +1479,11 @@ static Object* Runtime_StringCharAt(Arguments args) {
 
   CONVERT_CHECKED(String, subject, args[0]);
   Object* index = args[1];
-  return CharFromCode(CharCodeAt(subject, index));
+  Object* code = CharCodeAt(subject, index);
+  if (code == Heap::nan_value()) {
+    return Heap::undefined_value();
+  }
+  return CharFromCode(code);
 }