GHSA-6gv8-p3vj-pxvr

### Impact The code for `tf.raw_ops.UncompressElement` can be made to trigger a null pointer dereference: ```python import tensorflow as tf data = tf.data.Dataset.from_tensors([0.0]) tf.raw_ops.UncompressElement( compressed=tf.data.experimental.to_variant(data), output_types=[tf.int64], output_shapes=[2]) ``` The [implementation](https://github.com/tensorflow/tensorflow/blob/f24faa153ad31a4b51578f8181d3aaab77a1ddeb/tensorflow/core/kernels/data/experimental/compression_ops.cc#L50-L53) obtains a pointer to a `CompressedElement` from a `Variant` tensor and then proceeds to dereference it for decompressing. There is no check that the `Variant` tensor contained a `CompressedElement`, so the pointer is actually `nullptr`. ### Patches We have patched the issue in GitHub commit [7bdf50bb4f5c54a4997c379092888546c97c3ebd](https://github.com/tensorflow/tensorflow/commit/7bdf50bb4f5c54a4997c379092888546c97c3ebd). 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.