SPARK-2350: Don't NPE while launching drivers

Prior to this change, we could throw a NPE if we launch a driver while
another one is waiting, because removing from an iterator while iterating
over it is not safe.

core/src/main/scala/org/apache/spark/deploy/master/Master.scala

@@ -481,7 +481,7 @@ private[spark] class Master(
     // First schedule drivers, they take strict precedence over applications
     val shuffledWorkers = Random.shuffle(workers) // Randomization helps balance drivers
     for (worker <- shuffledWorkers if worker.state == WorkerState.ALIVE) {
-      for (driver <- waitingDrivers) {
+      for (driver <- List(waitingDrivers: _*)) { // iterate over a copy of waitingDrivers
         if (worker.memoryFree >= driver.desc.mem && worker.coresFree >= driver.desc.cores) {
           launchDriver(worker, driver)
           waitingDrivers -= driver