gh-96652: Fix faulthandler chained signal without sigaction() (#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.

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);