closes bpo-34474: Python/bltinmodule.c: Add missing NULL check in builtin_sum_impl() (GH-8872)

Reported by Svace static analyzer.

Python/bltinmodule.c

@@ -2401,6 +2401,11 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
                 }
             }
             result = PyFloat_FromDouble(f_result);
+            if (result == NULL) {
+                Py_DECREF(item);
+                Py_DECREF(iter);
+                return NULL;
+            }
             temp = PyNumber_Add(result, item);
             Py_DECREF(result);
             Py_DECREF(item);