HADOOP-16868. ipc.Server readAndProcess threw NullPointerException. Contributed by Tsz-wo Sze.

(cherry picked from commit cb3f3cca01d6ab9f4befb1bcd31f384c37c0231a)
(cherry picked from commit d411a849e10910d24cf1bbc8d64405470e00f510)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/Server.java

@@ -1789,7 +1789,7 @@ public abstract class Server {
 
     private SocketChannel channel;
     private ByteBuffer data;
-    private ByteBuffer dataLengthBuffer;
+    private final ByteBuffer dataLengthBuffer;
     private LinkedList<RpcCall> responseQueue;
     // number of outstanding rpcs
     private AtomicInteger rpcCount = new AtomicInteger();
@@ -2833,7 +2833,6 @@ public abstract class Server {
     private synchronized void close() {
       disposeSasl();
       data = null;
-      dataLengthBuffer = null;
       if (!channel.isOpen())
         return;
       try {socket.shutdownOutput();} catch(Exception e) {