fix byzzfuzz error when mutating after dropping

Author: joaomlneto

simulator/src/main/java/byzzbench/simulator/faults/behaviors/MutateMessageBehavior.java

@@ -66,6 +66,10 @@ public void accept(FaultContext context) {
 
         // apply the random mutator
         MessageMutationFault mutator = mutators.get(rand.nextInt(mutators.size()));
-        context.getScenario().getTransport().applyMutation(e.getEventId(), mutator);
+
+        // apply the mutation (if the message is still queued)
+        if (e.getStatus() == Event.Status.QUEUED) {
+            context.getScenario().getTransport().applyMutation(e.getEventId(), mutator);
+        }
     }
 }