Reference binding to nullptr in `MatrixSetDiagV*` ops
### Impact
An attacker can cause undefined behavior via binding a reference to null pointer in all operations of type `tf.raw_ops.MatrixSetDiagV*`:
```python
import tensorflow as tf
tf.raw_ops.MatrixSetDiagV3(
input=[1,2,3],
diagonal=[1,1],
k=[],
align='RIGHT_LEFT')
```
The implementation
https://github.com/tensorflow/tensorflow/blob/84d053187cb80d975ef2b9684d4b61981bca0c41/tensorflow/core/kernels/linalg/matrix_diag_op.cc has incomplete validation that the value of `k` is a valid tensor. We have check that this value is either a scalar or a vector, but there is no check for the number of elements. If this is an empty tensor, then code that accesses the first element of the tensor is wrong:
```cc
auto& diag_index = context->input(1);
...
lower_diag_index = diag_index.flat<int32>()(0);
```
### Patches
We have patched the issue in GitHub commit ff8894044dfae5568ecbf2ed514c1a37dc394f1b
https://github.com/tensorflow/tensorflow/commit/ff8894044dfae5568ecbf2ed514c1a37dc394f1b.
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.