Null pointer dereference in `StringNGrams`
### Impact
An attacker can trigger a dereference of a null pointer in `tf.raw_ops.StringNGrams`:
```python
import tensorflow as tf
data=tf.constant([''] * 11, shape=[11], dtype=tf.string)
splits = [0]*115
splits.append(3)
data_splits=tf.constant(splits, shape=[116], dtype=tf.int64)
tf.raw_ops.StringNGrams(data=data, data_splits=data_splits, separator=b'Ss',
ngram_widths=[7,6,11],
left_pad='ABCDE', right_pad=b'ZYXWVU',
pad_width=50, preserve_short_sequences=True)
```
This is because the implementation
https://github.com/tensorflow/tensorflow/blob/1cdd4da14282210cc759e468d9781741ac7d01bf/tensorflow/core/kernels/string_ngrams_op.cc#L67-L74 does not fully validate the `data_splits` argument. This would result in `ngrams_data`
https://github.com/tensorflow/tensorflow/blob/1cdd4da14282210cc759e468d9781741ac7d01bf/tensorflow/core/kernels/string_ngrams_op.cc#L106-L110 to be a null pointer when the output would be computed to have 0 or negative size.
Later writes to the output tensor would then cause a null pointer dereference.
### Patches
We have patched the issue in GitHub commit ba424dd8f16f7110eea526a8086f1a155f14f22b
https://github.com/tensorflow/tensorflow/commit/ba424dd8f16f7110eea526a8086f1a155f14f22b.
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 Yakun Zhang and Ying Wang of Baidu X-Team.