NPE in DFSOutputStream.closeBackupStream()

After HADOOP-757 is checked in, Devraj noticed following NPE:

java.lang.NullPointerException
at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.closeBackupStream(DFSClient.java:972)
at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.endBlock(DFSClient.java:1219)
at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.flush(DFSClient.java:1181)
at org.apache.hadoop.dfs.DFSClient$DFSOutputStream.write(DFSClient.java:1163)
at org.apache.hadoop.fs.FSDataOutputStream$Summer.write(FSDataOutputStream.java:85)
at org.apache.hadoop.fs.FSDataOutputStream$PositionCache.write(FSDataOutputStream.java:114)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
at java.io.DataOutputStream.flush(DataOutputStream.java:106)
at java.io.FilterOutputStream.close(FilterOutputStream.java:140)
at org.apache.hadoop.io.SequenceFile$Writer.close(SequenceFile.java:558)
at org.apache.hadoop.mapred.SequenceFileOutputFormat$1.close(SequenceFileOutputFormat.java:72)
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:331)
at org.apache.hadoop.mapred.TaskTracker$Child.main(TaskTracker.java:1367)

I will submit a patch after confirming its ok for backupStream to be null at this stage.