napi_get_value_string_X allow various kinds of memory corruption

`napi_get_value_string_latin1`, `napi_get_value_string_utf8`, `napi_get_value_string_utf16` are vulnerable to buffer overflows, partially due to an integer underflow.

Napi::Value Test(const Napi::CallbackInfo& info) {
  char buf[1];
  // This should be a valid call, e.g., due to a malloc(0).
  napi_get_value_string_latin1(info.Env(), info[0], buf, 0, nullptr);
  return info.Env().Undefined();
}