gh-99845: _PyObject_DictPointer(): fix dictoffset cast (#99922)

Cast size_t to Py_ssize_t, rather than casting it to long. On 64-bit
Windows, long is 32-bit whereas Py_ssize_t is 64-bit.

Author: vstinner

Diff for: Objects/object.c

@@ -1076,8 +1076,9 @@ _PyObject_DictPointer(PyObject *obj)
             tsize = -tsize;
         }
         size_t size = _PyObject_VAR_SIZE(tp, tsize);
+        assert(size <= (size_t)PY_SSIZE_T_MAX);
+        dictoffset += (Py_ssize_t)size;
 
-        dictoffset += (long)size;
         _PyObject_ASSERT(obj, dictoffset > 0);
         _PyObject_ASSERT(obj, dictoffset % SIZEOF_VOID_P == 0);
     }