Vulnerability: Null pointer dereference in `RaggedTensorToTensor` ### Impact
Sending invalid argument for `row_partition_types` of `tf.raw_ops.RaggedTensorToTensor` API results in a null pointer dereference and undefined behavior:

```python
import tensorflow as tf

tf.raw_ops.RaggedTensorToTensor(
  shape=1,
  values=10,
  default_value=21,
  row_partition_tensors=tf.constant([0,0,0,0]),
  row_partition_types=[])
```

The [implementation](https://github.com/tensorflow/tensorflow/blob/47a06f40411a69c99f381495f490536972152ac0/tensorflow/core/kernels/ragged_tensor_to_tensor_op.cc#L328) accesses the first element of a user supplied list of values without validating that the provided list is not empty.

### Patches
We have patched the issue in GitHub commit [301ae88b331d37a2a16159b65b255f4f9eb39314](https://github.com/tensorflow/tensorflow/commit/301ae88b331d37a2a16159b65b255f4f9eb39314).

The fix will be included in TensorFlow 2.6.0. We will also cherrypick this commit on TensorFlow 2.5.1, TensorFlow 2.4.3, and TensorFlow 2.3.4, as these are also affected and still in supported range.

### For more information
Please consult [our security guide](https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md) for more information regarding the security model and how to contact us with issues and questions.

### Attribution
This vulnerability has been reported by members of the Aivul Team from Qihoo 360.
Vulnerability ID: GHSA-hwr7-8gxx-fj5p
CWE: CWE-476
Severity score: 7.7
Project: https://github.com/tensorflow/tensorflow
Published: 2021-08-25
File: tensorflow/core/kernels/ragged_tensor_to_tensor_op.cc
Commit message: Fix null ptr deref in tf.raw_ops.RaggedTensorToTensor

PiperOrigin-RevId: 384257511
Change-Id: I0484ad285039d132d6c41b284a7fcdd2b774a38e
Commit author: Laura Pak

Vulnerable version (before fix):
 class RaggedTensorToTensorBaseOp : public OpKernel {   Status GetFirstDimensionSize(OpKernelContext* context, INDEX_TYPE* result) {     const Tensor first_partition_tensor =         context->input(kFirstPartitionInputIndex);     const RowPartitionType first_partition_type = row_partition_types_[0];     switch (first_partition_type) {       case RowPartitionType::FIRST_DIM_SIZE:

Fixed version (after fix):
 class RaggedTensorToTensorBaseOp : public OpKernel {   Status GetFirstDimensionSize(OpKernelContext* context, INDEX_TYPE* result) {     const Tensor first_partition_tensor =         context->input(kFirstPartitionInputIndex);     if (row_partition_types_.empty()) {       return errors::InvalidArgument("No row_partition_types given.");     }     const RowPartitionType first_partition_type = row_partition_types_[0];     switch (first_partition_type) {       case RowPartitionType::FIRST_DIM_SIZE:

Patch:
patch: @@ -348,6 +348,9 @@ class RaggedTensorToTensorBaseOp : public OpKernel {
   Status GetFirstDimensionSize(OpKernelContext* context, INDEX_TYPE* result) {
     const Tensor first_partition_tensor =
         context->input(kFirstPartitionInputIndex);
+    if (row_partition_types_.empty()) {
+      return errors::InvalidArgument("No row_partition_types given.");
+    }
     const RowPartitionType first_partition_type = row_partition_types_[0];
     switch (first_partition_type) {
       case RowPartitionType::FIRST_DIM_SIZE:
path: tensorflow/core/kernels/ragged_tensor_to_tensor_op.cc