Reference binding to nullptr in shape inference

Impact

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

```python

import tensorflow as tf

tf.compat.v1.disable_v2_behavior()

tf.raw_ops.SparseFillEmptyRows(

indices = tf.constant([], shape=[0, 0], dtype=tf.int64),

values = tf.constant([], shape=[0], dtype=tf.int64),

dense_shape = tf.constant([], shape=[0], dtype=tf.int64),

default_value = 0)

`

The shape inference [implementation](https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/core/ops/sparse_ops.cc#L608-L634) does not validate that the input arguments are not empty tensors.

Patches

We have patched the issue in GitHub commit [578e634b4f1c1c684d4b4294f9e5281b2133b3ed](https://github.com/tensorflow/tensorflow/commit/578e634b4f1c1c684d4b4294f9e5281b2133b3ed).

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 Yakun Zhang of Baidu Security