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.