index: drop support for nullid at position len(index) in index_node

I think no callers exist since at least a3dacabd476b (index: don't
allow index[len(index)] to mean nullid, 2018-07-20).

Differential Revision: https://phab.mercurial-scm.org/D4105

mercurial/cext/revlog.c

@@ -228,10 +228,10 b' static const char *index_node(indexObjec'
 	Py_ssize_t length = index_length(self) + 1;
 	const char *data;
 
-	if (pos == length - 1 || pos == -1)
+	if (pos == -1)
 		return nullid;
 
-	if (pos >= length)
+	if (pos >= length - 1)
 		return NULL;
 
 	if (pos >= self->length - 1) {