[fix] DigestManager should not advance readerIndex (#3919)

(cherry picked from commit df44920)

Author: michaeljmarshall

bookkeeper-server/src/main/java/org/apache/bookkeeper/proto/checksum/DigestManager.java

@@ -159,7 +159,7 @@ private ReferenceCounted computeDigestAndPackageForSendingV2(long entryId, long
         buf.readerIndex(0);
 
         if (isSmallEntry) {
-            buf.writeBytes(unwrapped);
+            buf.writeBytes(unwrapped, unwrapped.readerIndex(), unwrapped.readableBytes());
             unwrapped.release();
             return buf;
         } else {