Issue #7466: segmentation fault when the garbage collector is called
in the middle of populating a tuple.  Patch by Florent Xicluna.

(note: no NEWS entry for trunk since the bug was introduced in 2.7/3.1)
diff --git a/Objects/tupleobject.c b/Objects/tupleobject.c
index 0a9c718..384b830 100644
--- a/Objects/tupleobject.c
+++ b/Objects/tupleobject.c
@@ -875,7 +875,8 @@
 
 	/* XXX UNREF/NEWREF interface should be more symmetrical */
 	_Py_DEC_REFTOTAL;
-	_PyObject_GC_UNTRACK(v);
+	if (_PyObject_GC_IS_TRACKED(v))
+		_PyObject_GC_UNTRACK(v);
 	_Py_ForgetReference((PyObject *) v);
 	/* DECREF items deleted by shrinkage */
 	for (i = newsize; i < oldsize; i++) {