message: fix possible null ptr deref
  fixing getInputs() to always return Input* is painful as it requires multiple inheritance
  so let's keep it simple
  John's fix was the right one after all, sorry
diff: @@ -701,7 +701,8 @@ static void calculateAndInitConstants(Transform &t) {
  num_ptrinputs = 0;
  for (auto &arg : t.src.getInputs()) {
  auto n = num_ptrs(arg.getType());
  - if (dynamic_cast<const Input*>(&arg)->hasAttribute(ParamAttrs::ByVal)) {
  + auto in = dynamic_cast<const Input*>(&arg);
  + if (in && in->hasAttribute(ParamAttrs::ByVal)) {
  num_globals_src += n;
  num_globals += n;
  } else
  