HDFS-11476. Fix NPE in FsDatasetImpl#checkAndUpdate. Contributed by Xiaobing Zhou.

(cherry picked from commit ac5ae0065a127ac150a887fa6c6f3cffd86ef733)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java

@@ -2450,11 +2450,13 @@ class FsDatasetImpl implements FsDatasetSpi<FsVolumeImpl> {
         File memMetaFile = FsDatasetUtil.getMetaFile(diskFile,
             memBlockInfo.getGenerationStamp());
         if (fileIoProvider.exists(vol, memMetaFile)) {
-          if (memMetaFile.compareTo(diskMetaFile) != 0) {
-            LOG.warn("Metadata file in memory "
-                + memMetaFile.getAbsolutePath()
-                + " does not match file found by scan "
-                + (diskMetaFile == null? null: diskMetaFile.getAbsolutePath()));
+          String warningPrefix = "Metadata file in memory "
+              + memMetaFile.getAbsolutePath()
+              + " does not match file found by scan ";
+          if (!diskMetaFileExists) {
+            LOG.warn(warningPrefix + "null");
+          } else if (memMetaFile.compareTo(diskMetaFile) != 0) {
+            LOG.warn(warningPrefix + diskMetaFile.getAbsolutePath());
           }
         } else {
           // Metadata file corresponding to block in memory is missing