NullPointerException happens on XNIO ByteBufferSlicePool.clean() for non-direct buffers during the shutdown since EAP 7.2.4

Issue

When direct-buffers="false" is specified to <buffer-pool> in the io subsystem, the following NullPointerException happens during the shutdown since EAP 7.2.4.

10:02:08,180 WARN  [org.jboss.msc.service.fail] (MSC service thread 1-7) MSC000004: Failure during stop of service org.wildfly.io.buffer-pool.default: java.lang.NullPointerException
    at java.util.concurrent.ConcurrentLinkedQueue.addAll(ConcurrentLinkedQueue.java:530)
    at org.xnio.ByteBufferSlicePool.clean(ByteBufferSlicePool.java:214)
    at org.wildfly.extension.io.BufferPoolService.stop(BufferPoolService.java:62)
    at org.jboss.msc.service.ServiceControllerImpl$StopTask.stopService(ServiceControllerImpl.java:1795)
    at org.jboss.msc.service.ServiceControllerImpl$StopTask.execute(ServiceControllerImpl.java:1764)
    at org.jboss.msc.service.ServiceControllerImpl$ControllerTask.run(ServiceControllerImpl.java:1559)
    at org.jboss.threads.ContextClassLoaderSavingRunnable.run(ContextClassLoaderSavingRunnable.java:35)
    at org.jboss.threads.EnhancedQueueExecutor.safeRun(EnhancedQueueExecutor.java:1985)
    at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.doRunTask(EnhancedQueueExecutor.java:1487)
    at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1378)
    at java.lang.Thread.run(Thread.java:748)