[2.7] Issue GH-18560: Fix potential NULL pointer dereference in sum(). (GH-8892)

(cherry picked from commit 704e2d37)
Co-authored-by: Christian Heimes <christian@cheimes.de>

Misc/NEWS.d/next/Core and Builtins/2018-08-23-21-32-27.bpo-18560.5q_c1C.rst

+Fix potential NULL pointer dereference in sum().

Python/bltinmodule.c

@@ -2363,6 +2363,11 @@ builtin_sum(PyObject *self, PyObject *args)
             }
             /* Either overflowed or is not an int. Restore real objects and process normally */
             result = PyInt_FromLong(i_result);
+            if (result == NULL) {
+                Py_DECREF(item);
+                Py_DECREF(iter);
+                return NULL;
+            }
             temp = PyNumber_Add(result, item);
             Py_DECREF(result);
             Py_DECREF(item);