gh-101765: Fix refcount issues in list and unicode pickling (#102265)

Followup from #101769.

Author: JelleZijlstra

Objects/listobject.c

@@ -3451,16 +3451,24 @@ listiter_reduce_general(void *_it, int forward)
     /* the objects are not the same, index is of different types! */
     if (forward) {
         PyObject *iter = _PyEval_GetBuiltin(&_Py_ID(iter));
+        if (!iter) {
+            return NULL;
+        }
         _PyListIterObject *it = (_PyListIterObject *)_it;
         if (it->it_seq) {
             return Py_BuildValue("N(O)n", iter, it->it_seq, it->it_index);
         }
+        Py_DECREF(iter);
     } else {
         PyObject *reversed = _PyEval_GetBuiltin(&_Py_ID(reversed));
+        if (!reversed) {
+            return NULL;
+        }
         listreviterobject *it = (listreviterobject *)_it;
         if (it->it_seq) {
             return Py_BuildValue("N(O)n", reversed, it->it_seq, it->it_index);
         }
+        Py_DECREF(reversed);
     }
     /* empty iterator, create an empty list */
     list = PyList_New(0);

Objects/unicodeobject.c

@@ -14794,8 +14794,10 @@ unicodeiter_reduce(unicodeiterobject *it, PyObject *Py_UNUSED(ignored))
         return Py_BuildValue("N(O)n", iter, it->it_seq, it->it_index);
     } else {
         PyObject *u = unicode_new_empty();
-        if (u == NULL)
+        if (u == NULL) {
+            Py_DECREF(iter);
             return NULL;
+        }
         return Py_BuildValue("N(N)", iter, u);
     }
 }