nspawn: don't try to unregister a machine we never registered

When registering we condition this on "arg_register". Let's do the same
when unregistering, otherwise we might end up trying to unregister a
machine we never registered.

(cherry picked from commit 0790f4e)
(cherry picked from commit 6f346ef)
(cherry picked from commit 85804e6)

Author: poettering

src/nspawn/nspawn.c

@@ -5300,7 +5300,7 @@ static int run_container(
         r = wait_for_container(TAKE_PID(*pid), &container_status);
 
         /* Tell machined that we are gone. */
-        if (bus)
+        if (arg_register && bus)
                 (void) unregister_machine(bus, arg_machine);
 
         if (r < 0)