gh-96652: Fix faulthandler chained signal without sigaction() (GH-96666)


Fix the faulthandler implementation of faulthandler.register(signal,
chain=True) if the sigaction() function is not available: don't call
the previous signal handler if it's NULL.
(cherry picked from commit c580a81af91af4b9df85e466f8b48c3c9c86c3df)

Co-authored-by: Victor Stinner <vstinner@python.org>