Check buffer length using string length

+ Utf8Length is really slow but has the same semantics in this case

src/node_buffer.cc

@@ -406,7 +406,7 @@ Handle<Value> Buffer::Utf8Write(const Arguments &args) {
 
   size_t offset = args[1]->Uint32Value();
 
-  if (s->Utf8Length() > 0 && offset >= buffer->length_) {
+  if (s->Length() > 0 && offset >= buffer->length_) {
     return ThrowException(Exception::TypeError(String::New(
             "Offset is out of bounds")));
   }