[3.12] gh-121137: Add missing Py_DECREF calls for ADDITEMS opcode of _pickle.c (GH-121136) (#121140)

gh-121137: Add missing Py_DECREF calls for ADDITEMS opcode of _pickle.c (GH-121136)

PyObject_GetAttr returns a new reference, but this reference is never decremented using Py_DECREF, so Py_DECREF calls to this referece are added
(cherry picked from commit 92893fd)

Co-authored-by: Justin Applegate <[email protected]>

Author: miss-islington

Modules/_pickle.c

@@ -6623,11 +6623,13 @@ load_additems(PickleState *state, UnpicklerObject *self)
             if (result == NULL) {
                 Pdata_clear(self->stack, i + 1);
                 Py_SET_SIZE(self->stack, mark);
+                Py_DECREF(add_func);
                 return -1;
             }
             Py_DECREF(result);
         }
         Py_SET_SIZE(self->stack, mark);
+        Py_DECREF(add_func);
     }
 
     return 0;