#3663: extra DECREF on syntax errors.

Patch by Amaury Forgeot d'Arc, reviewed by Benjamin Peterson.

Misc/NEWS

@@ -12,6 +12,8 @@ What's New in Python 3.0 release candidate 1
 Core and Builtins
 -----------------
 
+- Issue #3663: Py_None was decref'd when printing SyntaxErrors.
+
 - Issue #3657: Fix uninitialized memory read when pickling longs.
   Found by valgrind.
 

Python/pythonrun.c

@@ -1242,7 +1242,10 @@ PyErr_PrintEx(int set_sys_last_vars)
 	if (exception == NULL)
 		return;
 	PyErr_NormalizeException(&exception, &v, &tb);
-	tb = tb ? tb : Py_None;
+	if (tb == NULL) {
+		tb = Py_None;
+		Py_INCREF(tb);
+	}
 	PyException_SetTraceback(v, tb);
 	if (exception == NULL)
 		return;