message: save undef vars after noreturn function call
  we need this for the noreturn SMT query
diff: @@ -208,6 +208,9 @@ void State::addUB(AndExpr &&ubs) {
  void State::addNoReturn() {
  function_domain.add(domain());
  + return_undef_vars.insert(undef_vars.begin(), undef_vars.end());
  + return_undef_vars.insert(domain.undef_vars.begin(), domain.undef_vars.end());
  + undef_vars.clear();
  addUB(expr(false));
  }
  