TestFsDatasetImpl missing null check when cleaning up

What happened

I turned on dfs.namenode.quota.init-threads=1468568631 and then the test org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.TestFsDatasetImpl#testMoveBlockFailurefails with null pointer.

Where's the problem

In the clean up part of the test:

    } finally {
      if (cluster.isClusterUp()) {
        cluster.shutdown();
      }
    }

if cluster is null, the test would directly fail with a null pointer exception and hiding potentially the actual failure.

How to reproduce

1. set dfs.namenode.quota.init-threads=1468568631 {}
2. run org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.TestFsDatasetImpl#testMoveBlockFailure
   you should observe

   java.lang.NullPointerException
       at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.TestFsDatasetImpl.testMoveBlockFailure(TestFsDatasetImpl.java:1005)

   For an easy reproduction, run the reproduce.sh in the attachment.

We are happy to provide a patch if this issue is confirmed.