logind: fix possible memleak of message if the message was already in the set

I'm not sure if it is actually possible to encounter this condition. But
let's make the handling correct regardless.

src/login/logind-brightness.c

@@ -175,9 +175,8 @@ static int set_add_message(Set **set, sd_bus_message *message) {
                 return r;
 
         r = set_ensure_put(set, &bus_message_hash_ops, message);
-        if (r < 0)
+        if (r <= 0)
                 return r;
-
         sd_bus_message_ref(message);
 
         return 1;