rcutorture: Gracefully handle NULL cleanup hooks

Although most torture tests will have some cleanup hook, it is possible
that one might not.  This commit therefore enables graceful handling of
a NULL cleanup hook during torture-test shutdown.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/torture.c

@@ -454,7 +454,10 @@ static int torture_shutdown(void *arg)
 
 	VERBOSE_TOROUT_STRING("torture_shutdown task shutting down system");
 	shutdown_task = NULL;	/* Avoid self-kill deadlock. */
-	torture_shutdown_hook();/* Shut down the enclosing torture test. */
+	if (torture_shutdown_hook)
+		torture_shutdown_hook();
+	else
+		VERBOSE_TOROUT_STRING("No torture_shutdown_hook(), skipping.");
 	kernel_power_off();	/* Shut down the system. */
 	return 0;
 }