CVE-2021-37647 Null pointer dereference in `SparseTensorSliceDataset` in TensorFlow

TensorFlow is an end-to-end open source platform for machine learning. When a user does not supply arguments that determine a valid sparse tensor, tf.raw_ops.SparseTensorSliceDataset implementation can be made to dereference a null pointer. The implementation has some argument validation but fails to consider the case when either indices or values are provided for an empty sparse tensor when the other is not. If indices is empty, then code that performs validation (i.e., checking that the indices are monotonically increasing) results in a null pointer dereference. If indices as provided by the user is empty, then indices in the C++ code above is backed by an empty std::vector, hence calling indices->dim_size(0) results in null pointer dereferencing (same as calling std::vector::at() on an empty vector). We have patched the issue in GitHub commit 02cc160e29d20631de3859c6653184e3f876b9d7. 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.