Fix edge case of extension with NULL as source string.

test/cctest/test-strings.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 
 // Check that we can traverse very deep stacks of ConsStrings using
 // StringInputBuffer.  Check that Get(int) works on very deep stacks
 // of ConsStrings.  These operations may not be very fast, but they
 // should be possible without getting errors due to too deep recursion.
 
 #include <stdlib.h>
 
 #include "v8.h"
 
 #include "api.h"
 #include "factory.h"
+#include "objects.h"
 #include "cctest.h"
 #include "zone-inl.h"
 
 unsigned int seed = 123;
 
 static uint32_t gen() {
         uint64_t z;
         z = seed;
         z *= 279470273;
         z %= 4294967291U;
@@ skipping 677 matching lines @@
       "var subject = 'ascii~only~string~'; "
       "var replace = '\x80';            "
       "subject.replace(/~/g, replace);  ");
   CHECK(result->IsString());
   Handle<String> string = v8::Utils::OpenHandle(v8::String::Cast(*result));
   CHECK(string->IsSeqTwoByteString());
 
   v8::Local<v8::String> expected = v8_str("ascii\x80only\x80string\x80");
   CHECK(expected->Equals(result));
 }
+
+
+TEST(IsAscii) {
+  CHECK(String::IsAscii(static_cast<char*>(NULL), 0));
+  CHECK(String::IsAscii(static_cast<uc16*>(NULL), 0));
+}