trying to fix NPE

not checked fully with compiler. At least it works in REPL:

scala> :paste

val system = new {
   lazy val a2: Actor.Address = actor {(m: Any) => if(m != "k") a1 ! "z" }
   lazy val a1: Actor.Address = actor {(m: Any) => if (m == "z") {println("z"); a2 ! "k"}}
}

scala> system.a2 ! "a"
z

scala> (1 to 100000).par.map{ _ => {system.a2 ! "a"}}
z
z
z
...