message: [tf.data / Cloud Bigtable]: Avoid underflow.
  Previously, because the size_t type is unsigned, if keys_.empty(), we would underflow, instead of returning end_of_sequence, resulting in a runtime crash. This change switches around the arithmetic so that it's robust to underflow.
diff: @@ -167,7 +167,7 @@ class BigtableSampleKeyPairsDatasetOp : public DatasetOpKernel {
  std::vector<Tensor>* out_tensors,
  bool* end_of_sequence) override {
  mutex_lock l(mu_);
  - if (index_ > keys_.size() - 2) {
  + if (index_ + 2 > keys_.size()) {
  *end_of_sequence = true;
  return Status::OK();
  }
  