Use NULL rather than 0. (#778)

There was few cases of using literal 0 instead of NULL in the context of
pointers.  While this was a legitimate C code, using NULL rather than 0 makes
the code clearer.

Modules/_cursesmodule.c

@@ -2987,7 +2987,7 @@ PyCurses_tigetstr(PyObject *self, PyObject *args)
         return NULL;
 
     capname = tigetstr( capname );
-    if (capname == 0 || capname == (char*) -1) {
+    if (capname == NULL || capname == (char*) -1) {
         Py_RETURN_NONE;
     }
     return PyBytes_FromString( capname );