CVE-2021-41198: Overflow/crash in `tf.tile` when tiling tensor is large

If tf.tile is called with a large input argument then the TensorFlow process will crash due to a CHECK-failure caused by an overflow.

import tensorflow as tf
import numpy as np
tf.keras.backend.tile(x=np.ones((1,1,1)), n=[100000000,100000000, 100000000])

The number of elements in the output tensor is too much for the int64_t type and the overflow is detected via a CHECK statement. This aborts the process.