bpo-36150: Fix possible assertion failures due to _ctypes.c's PyCData_reduce(). (GH-12106)

Modules/_ctypes/_ctypes.c

@@ -2743,10 +2743,11 @@ PyCData_reduce(PyObject *myself, PyObject *args)
                         "ctypes objects containing pointers cannot be pickled");
         return NULL;
     }
-    return Py_BuildValue("O(O(NN))",
-                         _unpickle,
-                         Py_TYPE(myself),
-                         PyObject_GetAttrString(myself, "__dict__"),
+    PyObject *dict = PyObject_GetAttrString(myself, "__dict__");
+    if (dict == NULL) {
+        return NULL;
+    }
+    return Py_BuildValue("O(O(NN))", _unpickle, Py_TYPE(myself), dict,
                          PyBytes_FromStringAndSize(self->b_ptr, self->b_size));
 }