message: fix UnreferencedInterfaces crash
  Summary: `type_class` can return null (on external classes for example)
diff: @@ -47,8 +47,10 @@ void get_interfaces(TypeSet& interfaces, DexClass* cls) {
  for (const auto& intf : intfs) {
  interfaces.insert(intf);
  const auto intf_cls = type_class(intf);
  + if (intf_cls != nullptr) {
  get_super_interfaces(interfaces, intf_cls);
  }
  + }
  const auto super = type_class(cls->get_super_class());
  if (super != nullptr) {
  get_interfaces(interfaces, super);
  