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 c580a81a)

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

Misc/NEWS.d/next/Library/2022-09-07-22-49-37.gh-issue-96652.YqOKxI.rst

+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. Patch by Victor Stinner.

Modules/faulthandler.c

@@ -862,7 +862,7 @@ faulthandler_user(int signum)
         errno = save_errno;
     }
 #else
-    if (user->chain) {
+    if (user->chain && user->previous != NULL) {
         errno = save_errno;
         /* call the previous signal handler */
         user->previous(signum);