HDFS-7101. Potential null dereference in DFSck#doWork(). Contributed by skrho and Ted Yu.

(cherry picked from commit 113af12cfb240ea9a7189bb2701693466eb8e993)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/tools/DFSck.java

@@ -354,7 +354,7 @@ public class DFSck extends Configured implements Tool {
     BufferedReader input = new BufferedReader(new InputStreamReader(
                                               stream, "UTF-8"));
     String line = null;
-    String lastLine = null;
+    String lastLine = NamenodeFsck.CORRUPT_STATUS;
     int errCode = -1;
     try {
       while ((line = input.readLine()) != null) {