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.