message: fix getBaseContext for null
diff: @@ -396,7 +396,7 @@ Plugin::RootContext* NullVmPlugin::ensureRootContext(uint64_t context_id,
  Plugin::ContextBase* NullVmPlugin::getContextBase(uint64_t context_id) {
  auto it = context_map_.find(context_id);
  - if (it == context_map_.end() || !it->second->asContext()) {
  + if (it == context_map_.end() || !(it->second->asContext() || it->second->asRoot())) {
  throw WasmException("no base context context_id: " + std::to_string(context_id));
  }
  return it->second.get();
  