Avoid NPE in Datanode heartbeat

2014-02-05 17:24:11,347 WARN org.apache.hadoop.ipc.Server: IPC Server handler 5 on 9000, call org.apache.hadoop.hdfs.server.protocol.DatanodeProtocol.sendHeartbeat from 10.18.40.32:13299 Call#1721 Retry#0: error: java.lang.NullPointerException
java.lang.NullPointerException
	at org.apache.hadoop.hdfs.protocolPB.PBHelper.convertRollingUpgradeStatus(PBHelper.java:1460)
	at org.apache.hadoop.hdfs.protocolPB.DatanodeProtocolServerSideTranslatorPB.sendHeartbeat(DatanodeProtocolServerSideTranslatorPB.java:124)
	at org.apache.hadoop.hdfs.protocol.proto.DatanodeProtocolProtos$DatanodeProtocolService$2.callBlockingMethod(DatanodeProtocolProtos.java:27704)
	at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:605)
	at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:932)
	at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2048)