Translog: make sure stats's op count and size are in sync

During translog flush there is a very small window where translog stats reports can be inconsistent.

Closes #10041

src/main/java/org/elasticsearch/index/translog/fs/FsTranslog.java

@@ -439,7 +439,12 @@ public class FsTranslog extends AbstractIndexShardComponent implements Translog
 
     @Override
     public TranslogStats stats() {
-        return new TranslogStats(estimatedNumberOfOperations(), translogSizeInBytes());
+        FsTranslogFile current = this.current;
+        if (current == null) {
+            return new TranslogStats(0, 0);
+        }
+
+        return new TranslogStats(current.estimatedNumberOfOperations(), current.translogSizeInBytes());
     }
 
     @Override