HDFS-8548. Minicluster throws NPE on shutdown. Contributed by surendra singh lilhore.

(cherry picked from commit 6a76250b39f33466bdc8dabab33070c90aa1a389)

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -560,6 +560,9 @@ Release 2.8.0 - UNRELEASED
 
     HDFS-8592. SafeModeException never get unwrapped. (wheat9)
 
+    HDFS-8548. Minicluster throws NPE on shutdown.
+    (surendra singh lilhore via xyao)
+
 Release 2.7.1 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlocksMap.java

@@ -198,7 +198,11 @@ class BlocksMap {
   }
 
   int size() {
-    return blocks.size();
+    if (blocks != null) {
+      return blocks.size();
+    } else {
+      return 0;
+    }
   }
 
   Iterable<BlockInfo> getBlocks() {