CVE-2021-41217: Null pointer exception when `Exit` node is not preceded by `Enter` op

The process of building the control flow graph for a TensorFlow model is vulnerable to a null pointer exception when nodes that should be paired are not:

import tensorflow as tf

@tf.function
def func():
return tf.raw_ops.Exit(data=[False,False])

func()

This occurs because the code assumes that the first node in the pairing (e.g., an Enter node) always exists when encountering the second node (e.g., an Exit node):

...
} else if (IsExit(curr_node)) {
// Exit to the parent frame.
parent = parent_nodes[curr_id];
frame_name = cf_info->frame_names[parent->id()];
...

When this is not the case, parent is nullptr so dereferencing it causes a crash.