Null pointer dereference in `CompressElement`

Impact

It is possible to trigger a null pointer dereference in TensorFlow by passing an invalid input to tf.raw_ops.CompressElement:

```python

import tensorflow as tf

tf.raw_ops.CompressElement(components=[[]])

`

The [implementation](https://github.com/tensorflow/tensorflow/blob/47a06f40411a69c99f381495f490536972152ac0/tensorflow/core/data/compression_utils.cc#L34) was accessing the size of a buffer obtained from the return of a separate function call before validating that said buffer is valid.

Patches

We have patched the issue in GitHub commit [5dc7f6981fdaf74c8c5be41f393df705841fb7c5](https://github.com/tensorflow/tensorflow/commit/5dc7f6981fdaf74c8c5be41f393df705841fb7c5).

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. Concurrently, it was resolved in master branch as it was also discovered internally and fixed before the report was handled.