message: fix ubsan error
diff: @@ -61,7 +61,7 @@ public:
  Function(Type &type, std::string &&name)
  : type(&type), name(std::move(name)) {}
  - const IR::Type& getType() const { return *type; }
  + const IR::Type& getType() const { return type ? *type : Type::voidTy; }
  void setType(IR::Type &t) { type = &t; }
  smt::expr getTypeConstraints() const;
  