Fix runtime function StringCharAt where an argument was not checked.

src/runtime.cc

 // Copyright 2006-2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1390 matching lines @@
 
   CONVERT_CHECKED(String, subject, args[0]);
   Object* index = args[1];
   return CharCodeAt(subject, index);
 }
 
 
 static Object* Runtime_StringCharAt(Arguments args) {
   NoHandleAllocation ha;
   ASSERT(args.length() == 2);
-  return CharFromCode(Runtime_StringCharCodeAt(args));
+
+  CONVERT_CHECKED(String, subject, args[0]);
+  Object* index = args[1];
+  return CharFromCode(CharCodeAt(subject, index));
 }
 
 
 static Object* Runtime_CharFromCode(Arguments args) {
   NoHandleAllocation ha;
   ASSERT(args.length() == 1);
   return CharFromCode(args[0]);
 }
 
 // Forward declarations.
@@ skipping 6617 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     Counters::gc_last_resort_from_js.Increment();
     Heap::CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal