word-graph: fix follow_path return type

Author: james-d-mitchell

src/word-graph.cpp

@@ -760,7 +760,7 @@ The word graphs *x* and *y* are equal at a node *s* if:
     m.def(
         "word_graph_follow_path",
         [](WordGraph_ const& wg, node_type from, word_type const& path) {
-          return word_graph::follow_path(wg, from, path);
+          return from_int(word_graph::follow_path(wg, from, path));
         },
         py::arg("wg"),
         py::arg("from"),

tests/test_word_graph.py

@@ -137,6 +137,9 @@ def test_follow_path(word_graphs):
     with pytest.raises(LibsemigroupsError):
         word_graph.follow_path(wg1, 6, [0])
 
+    wg1.init(1, 2)
+    assert word_graph.follow_path(wg1, 0, [0] * 6) == UNDEFINED
+
 
 def test_is_acyclic(word_graphs):
     wg1, wg2 = word_graphs