tests: fix invalid assignment when using variants in C++11

tests: fix invalid assignment when using variants in C++11

* tests/c++.at (Exception safety): In variant mode $$ is an instance
of Object.  Assigning YY_NULL in C++98 is incorrect, but behaves ok,
as it assigns YY_NULL=0 using Object::operator= (char v).  It is wrong
in C++11 as there is operator for "$$ = nullptr".