core: fix the return value in order to make sure we don't dipatch method return too early

Actually, it is the same kind of problem as in d910f4c . Basically, we
need to return 1 on success code path in slice_freezer_action().
Otherwise we dispatch DBus return message too soon.

Fixes: #16050

src/core/slice.c

@@ -403,7 +403,7 @@ static int slice_freezer_action(Unit *s, FreezerAction action) {
         if (r < 0)
                 return r;
 
-        return 0;
+        return 1;
 }
 
 static int slice_freeze(Unit *s) {