message: fix crash in vector extract with an invalid vector
diff: @@ -749,6 +749,9 @@ VectorType::VectorType(string &&name, unsigned elements, Type &elementTy)
  StateValue VectorType::extract(const StateValue &vector,
  const expr &index) const {
  + if (!vector.isValid())
  + return {};
  +
  auto &elementTy = *children[0];
  unsigned bw_elem = elementTy.bits();
  unsigned bw_val = vector.bits();
  