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

Fix the following calls: _curses.tigetflag(None), _curses.tigetnum(None),
_curses.tigetstr(None).
diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
index 37ba95d..430ab17 100644
--- a/Modules/_cursesmodule.c
+++ b/Modules/_cursesmodule.c
@@ -2466,7 +2466,7 @@
 
 	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 @@
 
 	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 @@
 
 	PyCursesSetupTermCalled;
 		
-	if (!PyArg_ParseTuple(args, "z", &capname))
+	if (!PyArg_ParseTuple(args, "s", &capname))
 		return NULL;
 
 	capname = tigetstr( capname );