add NullMessage after Supervise to prevent race, see #2418

- parent just checked for system messages and finds none
- Supervise is enqueued to parent
- Failed is enqueued to the parent
- Failed is processed before Supervise and thus dropped due to
  non-matching UID

remedy is to enqueue NullMessage after enqueuing Supervise but before
really starting the actor, so that if NullMessage overtakes Supervise
the loop in processMailbox will then pick up Supervise next

akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala

@@ -37,6 +37,13 @@ object Envelope {
   }
 }
 
+/**
+ * This message is sent directly after the Supervise system message in order
+ * to form a barrier wrt. the first real message sent by the child, so that e.g.
+ * Failed() cannot overtake Supervise(). Processing this does nothing.
+ */
+case object NullMessage extends AutoReceivedMessage
+
 /**
  * INTERNAL API
  */