bpo-38321: Fix PyCStructUnionType_update_stgdict() warning (GH-16492)

bpo-22273, bpo-38321: Fix following warning:

    modules\_ctypes\stgdict.c(704):
    warning C4244: 'initializing': conversion from 'Py_ssize_t' to 'int', possible loss of data

Modules/_ctypes/stgdict.c

@@ -701,7 +701,7 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
                 assert(actual_type_index <= MAX_ELEMENTS);
             }
             else {
-                int length = dict->length;
+                Py_ssize_t length = dict->length;
                 StgDictObject *edict;
 
                 edict = PyType_stgdict(dict->proto);