Clear leader data when mode goes to detached. (#1809)

The effect of not clearing leader data in detached state is, when the
device attaches back, network data is never requested when leader data
is handled.

src/core/thread/mle.cpp

@@ -261,6 +261,7 @@ otError Mle::Stop(bool aClearNetworkDatasets)
     mNetif.RemoveUnicastAddress(mMeshLocal16);
     mNetif.GetNetworkDataLocal().Clear();
     mNetif.GetNetworkDataLeader().Clear();
+    memset(&mLeaderData, 0, sizeof(mLeaderData));
 
     if (aClearNetworkDatasets)
     {