_PyObject_FastCallKeywords() now checks !PyErr_Occurred()

Issue python#29259. All other functions calling functions start with the similar
assertion.

Author: vstinner

Objects/abstract.c

@@ -2482,6 +2482,11 @@ PyObject *
 _PyObject_FastCallKeywords(PyObject *callable, PyObject **stack, Py_ssize_t nargs,
                            PyObject *kwnames)
 {
+    /* _PyObject_FastCallKeywords() must not be called with an exception set,
+       because it can clear it (directly or indirectly) and so the
+       caller loses its exception */
+    assert(!PyErr_Occurred());
+
     assert(nargs >= 0);
     assert(kwnames == NULL || PyTuple_CheckExact(kwnames));