fix: prevent segmentation fault

[sceneq]

Fix the segmentation fault that occurs with the following code:

PY_LANGUAGE = Language(tspython.language(), "python") # type: ignore
parser = Parser()
parser.set_language(PY_LANGUAGE)

tree = parser.parse(b"i = 0")
node = tree.root_node
v = node.child(node.child_count)

print(v)  # This line will cause a SIGSEGV

assert v is None

This commit corresponds to this code in the Rust binding

[ObserverOfTime]

It will be fixed by #208