[network-data] retrieve network data when restoring as leader (#3354)

The leader does not maintain network data in non-volatile memeory. As a
result, it is necessary for a recently-restored leader to obtain network
data from a neighboring device.

src/core/thread/mle.cpp

@@ -1255,6 +1255,8 @@ otError Mle::AppendNetworkData(Message &aMessage, bool aStableOnly)
     otError        error = OT_ERROR_NONE;
     NetworkDataTlv tlv;
 
+    VerifyOrExit(!mRetrieveNewNetworkData, error = OT_ERROR_INVALID_STATE);
+
     tlv.Init();
     FillNetworkDataTlv(tlv, aStableOnly);
 

src/core/thread/mle_router.cpp

@@ -936,10 +936,11 @@ otError MleRouter::HandleLinkAccept(const Message &         aMessage,
         else
         {
             SetStateRouter(GetRloc16());
-            mRetrieveNewNetworkData = true;
-            SendDataRequest(aMessageInfo.GetPeerAddr(), dataRequestTlvs, sizeof(dataRequestTlvs), 0);
         }
 
+        mRetrieveNewNetworkData = true;
+        SendDataRequest(aMessageInfo.GetPeerAddr(), dataRequestTlvs, sizeof(dataRequestTlvs), 0);
+
 #if OPENTHREAD_CONFIG_ENABLE_TIME_SYNC
         GetNetif().GetTimeSync().HandleTimeSyncMessage(aMessage);
 #endif