buffer: removing duplicate code - #1144

- using an overload of Alloc that does the same that was being done
  inside `Buffer::New`

The overload we now call inside `smalloc.cc` takes care of the same as
the code that was removed:

    if (length == 0)
      return Alloc(env, obj, nullptr, length, type);

    char* data = static_cast<char*>(malloc(length));
    if (data == nullptr) {
      FatalError("node::smalloc::Alloc(v8::Handle<v8::Object>, size_t,"
                  " v8::ExternalArrayType)", "Out Of Memory");
    }

    Alloc(env, obj, data, length, type);

PR-URL: #1144
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>