[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.

PiperOrigin-RevId: 219062746

tensorflow/contrib/bigtable/kernels/bigtable_sample_key_pairs_dataset_op.cc

@@ -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();
         }