message: fix crash with void functions
diff: @@ -208,7 +208,7 @@ void VoidType::fixup(const Model &m) {
  pair<expr, expr>
  VoidType::refines(const StateValue &src, const StateValue &tgt) const {
  - UNREACHABLE();
  + return { true, true };
  }
  pair<expr, vector<expr>> VoidType::mkInput(State &s, const char *name) const {
  