adds in null checks for tracker

Author: ahmarsuhail

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/common/CachingBlockManager.java

@@ -348,7 +348,7 @@ private void readBlock(BufferData data, boolean isPrefetch, BufferData.State...
         String message = String.format("error during readBlock(%s)", data.getBlockNumber());
         LOG.error(message, e);
 
-        if(isPrefetch) {
+        if (isPrefetch && tracker != null) {
           tracker.failed();
         }
 
@@ -362,7 +362,9 @@ private void readBlock(BufferData data, boolean isPrefetch, BufferData.State...
 
         if (isPrefetch) {
           prefetchingStatistics.prefetchOperationCompleted();
-          tracker.close();
+          if (tracker != null) {
+            tracker.close();
+          }
         }
       }
     }