Small improvement to the Python kore.timer() api.

Do not allow kore.timer() to be called from the parent process
as it shouldn't be run there.

This makes Kore fail more gracefully.

src/python.c

@@ -2441,6 +2441,12 @@ python_kore_timer(PyObject *self, PyObject *args, PyObject *kwargs)
 	int			flags;
 	struct pytimer		*timer;
 
+	if (worker == NULL) {
+		PyErr_SetString(PyExc_RuntimeError,
+		    "kore.timer not supported on parent process");
+		return (NULL);
+	}
+
 	if (!PyArg_ParseTuple(args, "OKi", &obj, &ms, &flags))
 		return (NULL);