Avoid pointer underflow in CopyCharsUnsigned.

test/cctest/test-api.cc

 // Copyright 2012 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 6185 matching lines @@
   len = str3->WriteAscii(buf);
   CHECK_EQ(7, len);
   CHECK_EQ(0, strcmp("abc def", buf));
 
   memset(buf, 0x1, sizeof(buf));
   len = str3->WriteAscii(buf, 0, -1, String::PRESERVE_ASCII_NULL);
   CHECK_EQ(7, len);
   CHECK_EQ(0, strcmp("abc", buf));
   CHECK_EQ(0, buf[3]);
   CHECK_EQ(0, strcmp("def", buf + 4));
+
+  CHECK_EQ(0, str->WriteAscii(NULL, 0, 0, String::NO_NULL_TERMINATION));
+  CHECK_EQ(0, str->WriteUtf8(NULL, 0, 0, String::NO_NULL_TERMINATION));
+  CHECK_EQ(0, str->Write(NULL, 0, 0, String::NO_NULL_TERMINATION));
 }
 
 
 static void Utf16Helper(
     LocalContext& context,
     const char* name,
     const char* lengths_name,
     int len) {
   Local<v8::Array> a =
       Local<v8::Array>::Cast(context->Global()->Get(v8_str(name)));
@@ skipping 11904 matching lines @@
   };
 
   i::Semaphore* sem_;
   volatile int sem_value_;
 };
 
 
 THREADED_TEST(SemaphoreInterruption) {
   ThreadInterruptTest().RunTest();
 }
+
 #endif  // WIN32