RFR 8157528: jdk/jshell/ExecutionControlTest.java failed intermittently with NPE

[Robert Field]

Bug:   https://bugs.openjdk.java.net/browse/JDK-8157528
      


                  snippetThread.start();
                  Thread[] threadList = new Thread[execThreadGroup.activeCount()];
                  execThreadGroup.enumerate(threadList);
                  for (Thread thread : threadList) {
+                    if (thread != null)
                      thread.join();
                  }
  
                  if (stopped.get()) {
                      debug("Killed.");