Use also buffered_CAST() in buffered_iternext()

Author: vstinner

Modules/_io/bufferedio.c

@@ -1484,7 +1484,7 @@ _io__Buffered_truncate_impl(buffered *self, PyTypeObject *cls, PyObject *pos)
 static PyObject *
 buffered_iternext(PyObject *op)
 {
-    buffered *self = (buffered*)op;
+    buffered *self = buffered_CAST(op);
     PyObject *line;
     PyTypeObject *tp;