Fix _curses.tiget*() functions: deny None to avoid a crash.

Fix the following calls: _curses.tigetflag(None), _curses.tigetnum(None),
_curses.tigetstr(None).

Modules/_cursesmodule.c

@@ -2466,7 +2466,7 @@ PyCurses_tigetflag(PyObject *self, PyObject *args)
 
 	PyCursesSetupTermCalled;
 		
-	if (!PyArg_ParseTuple(args, "z", &capname))
+	if (!PyArg_ParseTuple(args, "s", &capname))
 		return NULL;
 
 	return PyInt_FromLong( (long) tigetflag( capname ) );
@@ -2479,7 +2479,7 @@ PyCurses_tigetnum(PyObject *self, PyObject *args)
 
 	PyCursesSetupTermCalled;
 		
-	if (!PyArg_ParseTuple(args, "z", &capname))
+	if (!PyArg_ParseTuple(args, "s", &capname))
 		return NULL;
 
 	return PyInt_FromLong( (long) tigetnum( capname ) );
@@ -2492,7 +2492,7 @@ PyCurses_tigetstr(PyObject *self, PyObject *args)
 
 	PyCursesSetupTermCalled;
 		
-	if (!PyArg_ParseTuple(args, "z", &capname))
+	if (!PyArg_ParseTuple(args, "s", &capname))
 		return NULL;
 
 	capname = tigetstr( capname );