Issue #4653: fix typo in flush_std_files()

Don't call sys.stderr.flush() if sys has no stderr attribute or if
sys.stderr==None.
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index e58b1c8..3031aef 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -334,7 +334,7 @@
             Py_DECREF(tmp);
     }
 
-    if (ferr != NULL || ferr != Py_None) {
+    if (ferr != NULL && ferr != Py_None) {
         tmp = PyObject_CallMethod(ferr, "flush", "");
         if (tmp == NULL)
             PyErr_Clear();