broker: fix leak in broker_online_cb

Problem: An idset is leaked in broker_online_cb().

Destroy the leaked idset.

Author: grondo

src/broker/state_machine.c

@@ -923,6 +923,7 @@ static void broker_online_cb (flux_future_t *f, void *arg)
             free (hosts);
             free (ranks);
         }
+        idset_destroy (loss);
     }
 
     idset_destroy (previous_online);