Null pointer exception in `DeserializeSparse`
### Impact
The shape inference code for `DeserializeSparse`
https://github.com/tensorflow/tensorflow/blob/8d72537c6abf5a44103b57b9c2e22c14f5f49698/tensorflow/core/ops/sparse_ops.cc#L152-L168 can trigger a null pointer dereference:
```python
import tensorflow as tf
dataset = tf.data.Dataset.range(3)
@tf.function
def test():
y = tf.raw_ops.DeserializeSparse(
serialized_sparse=tf.data.experimental.to_variant(dataset),
dtype=tf.int32)
test()
```
This is because the shape inference function assumes that the `serialize_sparse` tensor is a tensor with positive rank (and having `3` as the last dimension). However, in the example above, the argument is a scalar (i.e., rank 0).
### Patches
We have patched the issue in GitHub commit d3738dd70f1c9ceb547258cbb82d853da8771850
https://github.com/tensorflow/tensorflow/commit/d3738dd70f1c9ceb547258cbb82d853da8771850.
The fix will be included in TensorFlow 2.7.0. We will also cherrypick this commit on TensorFlow 2.6.1, TensorFlow 2.5.2, and TensorFlow 2.4.4, 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.