Fix edge case of extension with NULL as source string.

test/cctest/test-strings.cc

Index: test/cctest/test-strings.cc
diff --git a/test/cctest/test-strings.cc b/test/cctest/test-strings.cc
index 4557100e7ab1d1ec288d1135d0986ffb8264093c..5a9ccbb5790732cb0b42528860867bcbafa811be 100644
--- a/test/cctest/test-strings.cc
+++ b/test/cctest/test-strings.cc
@@ -11,6 +11,7 @@
 
 #include "api.h"
 #include "factory.h"
+#include "objects.h"
 #include "cctest.h"
 #include "zone-inl.h"
 
@@ -708,3 +709,9 @@ TEST(StringReplaceAtomTwoByteResult) {
   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));
+}