merge -r59315:59316 from py3k: Fix issue #1553: An errornous __length_hint__...

merge -r59315:59316 from py3k: Fix issue #1553: An errornous __length_hint__ can make list() raise a SystemError

Objects/abstract.c

@@ -1507,8 +1507,9 @@ PySequence_Tuple(PyObject *v)
 	/* Guess result size and allocate space. */
 	n = _PyObject_LengthHint(v);
 	if (n < 0) {
-		if (!PyErr_ExceptionMatches(PyExc_TypeError)  &&
-		    !PyErr_ExceptionMatches(PyExc_AttributeError)) {
+		if (PyErr_Occurred()
+		    && !PyErr_ExceptionMatches(PyExc_TypeError)
+		    && !PyErr_ExceptionMatches(PyExc_AttributeError)) {
 			Py_DECREF(it);
 			return NULL;
 		}

Objects/listobject.c

@@ -796,8 +796,9 @@ listextend(PyListObject *self, PyObject *b)
 	/* Guess a result list size. */
 	n = _PyObject_LengthHint(b);
 	if (n < 0) {
-		if (!PyErr_ExceptionMatches(PyExc_TypeError)  &&
-		    !PyErr_ExceptionMatches(PyExc_AttributeError)) {
+		if (PyErr_Occurred()
+		    && !PyErr_ExceptionMatches(PyExc_TypeError)
+		    && !PyErr_ExceptionMatches(PyExc_AttributeError)) {
 			Py_DECREF(it);
 			return NULL;
 		}