message: Fix a bug which may cause segmentation fault
diff: @@ -1586,6 +1586,10 @@ class HoistCWiseUnaryChainsStage : public ArithmeticOptimizerStage {
  const int n = node.attr().at("N").i();
  const int start = node.op() == "Concat" ? 1 : 0;
  const int end = start + n;
  + if (end > node.input_size()) {
  + return errors::FailedPrecondition(
  + "Got attr N=", n, " without enough inputs.");
  + }
  // Set up tail pointers to point to the immediate inputs to Concat.
  for (int input_port = start; input_port < end; ++input_port) {
  if (IsControlInput(node.input(input_port))) {
  