Crash when there are less vertices than workers

[gdiscry]

When there are less vertices than workers, Graph.iterate crashes.

processing.parallel.Graph@b05236: caught java.lang.IllegalArgumentException: requirement failed
java.lang.IllegalArgumentException: requirement failed
    at scala.Predef$.require(Predef.scala:133)
    at scala.collection.Iterator$GroupedIterator.<init>(Iterator.scala:773)
    at scala.collection.Iterator$class.grouped(Iterator.scala:886)
    at scala.collection.LinearSeqLike$$anon$1.grouped(LinearSeqLike.scala:52)
    at scala.collection.IterableLike$class.grouped(IterableLike.scala:147)
    at scala.collection.immutable.List.grouped(List.scala:45)
    at processing.parallel.Graph.createWorkers(ProcessingParallel.scala:135)
    at processing.parallel.Graph$$anonfun$act$1$$anonfun$apply$mcV$sp$1.apply(ProcessingParallel.scala:184)
    at processing.parallel.Graph$$anonfun$act$1$$anonfun$apply$mcV$sp$1.apply(ProcessingParallel.scala:178)
    at scala.actors.Actor$class.receive(Actor.scala:524)
    at processing.parallel.Graph.receive(ProcessingParallel.scala:98)
    at processing.parallel.Graph$$anonfun$act$1.apply$mcV$sp(ProcessingParallel.scala:178)
    at processing.parallel.Graph$$anonfun$act$1.apply(ProcessingParallel.scala:178)
    at processing.parallel.Graph$$anonfun$act$1.apply(ProcessingParallel.scala:178)
    at scala.actors.Reactor$class.seq(Reactor.scala:280)
    at processing.parallel.Graph.seq(ProcessingParallel.scala:98)
    at scala.actors.Reactor$$anon$3.andThen(Reactor.scala:258)
    at scala.actors.Combinators$class.loop(Combinators.scala:26)
    at processing.parallel.Graph.loop(ProcessingParallel.scala:98)
    at processing.parallel.Graph.act(ProcessingParallel.scala:177)
    at scala.actors.Reactor$$anonfun$dostart$1.apply(Reactor.scala:222)
    at scala.actors.Reactor$$anonfun$dostart$1.apply(Reactor.scala:222)
    at scala.actors.ReactorTask.run(ReactorTask.scala:36)
    at scala.concurrent.forkjoin.ForkJoinPool$AdaptedRunnable.exec(ForkJoinPool.java:611)
    at scala.concurrent.forkjoin.ForkJoinTask.quietlyExec(ForkJoinTask.java:422)
    at scala.concurrent.forkjoin.ForkJoinWorkerThread.mainLoop(ForkJoinWorkerThread.java:340)
    at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:325)