Reference binding to nullptr in `RaggedTensorToSparse`

Impact

An attacker can cause undefined behavior via binding a reference to null pointer in tf.raw_ops.RaggedTensorToSparse:

```python

import tensorflow as tf

tf.raw_ops.RaggedTensorToSparse(

rt_nested_splits=[[0, 38, 0]],

rt_dense_values=[])

`

The [implementation](https://github.com/tensorflow/tensorflow/blob/f24faa153ad31a4b51578f8181d3aaab77a1ddeb/tensorflow/core/kernels/ragged_tensor_to_sparse_kernel.cc#L30) has an incomplete validation of the splits values: it does not check that they are in increasing order.

Patches

We have patched the issue in GitHub commit [1071f554dbd09f7e101324d366eec5f4fe5a3ece](https://github.com/tensorflow/tensorflow/commit/1071f554dbd09f7e101324d366eec5f4fe5a3ece).

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.