CVE-2021-41196: Crash in `max_pool3d` when size argument is 0 or negative

The Keras pooling layers can trigger a segfault if the size of the pool is 0 or if a dimension is negative:

import tensorflow as tf

pool_size = [2, 2, 0]
layer = tf.keras.layers.MaxPooling3D(strides=1, pool_size=pool_size)
input_tensor = tf.random.uniform([3, 4, 10, 11, 12], dtype=tf.float32)
res = layer(input_tensor)

This is due to the TensorFlow’s implementation of pooling operations where the values in the sliding window are not checked to be strictly positive.