Fix retainment of shared reference count.

Author: ZeroIntensity

Objects/object.c

@@ -2541,7 +2541,7 @@ PyUnstable_Object_EnableDeferredRefcount(PyObject *op)
         return 0;
     }
     Py_ssize_t shared = _Py_atomic_load_ssize(&op->ob_ref_shared);
-    _Py_atomic_store_ssize(&op->ob_ref_shared, _Py_REF_SHARED(_Py_REF_DEFERRED, shared));
+    _Py_atomic_store_ssize(&op->ob_ref_shared, _Py_REF_SHARED(_Py_REF_DEFERRED, 0) + shared);
     return 1;
 #else
     return 0;