add clearing of message from tail node to avoid memory leaks when que…

…ue is empty and no new messages come

concurrent/src/main/scala/scalaz/concurrent/Actor.scala

@@ -56,6 +56,7 @@ final case class Actor[A](handler: A => Unit, onError: Throwable => Unit = throw
     val t = tail.get
     val n = batchHandle(t, 1024)
     if (n ne t) {
+      n.a = null.asInstanceOf[A]
       tail.lazySet(n)
       schedule()
     } else {
@@ -78,7 +79,7 @@ final case class Actor[A](handler: A => Unit, onError: Throwable => Unit = throw
   }
 }
 
-private class Node[A](val a: A = null.asInstanceOf[A]) extends AtomicReference[Node[A]]
+private class Node[A](var a: A = null.asInstanceOf[A]) extends AtomicReference[Node[A]]
 
 object Actor extends ActorFunctions with ActorInstances