fix sed issue - unable to reattach when the parent is removed (#326)

src/core/thread/mesh_forwarder.cpp

@@ -957,6 +957,7 @@ void MeshForwarder::HandleSentFrame(Mac::Frame &aFrame)
 {
     Mac::Address macDest;
     Child *child;
+    Neighbor *neighbor;
 
     mSendBusy = false;
 
@@ -1004,6 +1005,17 @@ void MeshForwarder::HandleSentFrame(Mac::Frame &aFrame)
         }
     }
 
+    if (mSendMessage->GetType() == Message::kTypeMacDataPoll)
+    {
+        neighbor = mMle.GetParent();
+
+        if (neighbor->mState == Neighbor::kStateInvalid)
+        {
+            mPollTimer.Stop();
+            mMle.BecomeDetached();
+        }
+    }
+
     if (mSendMessage->GetDirectTransmission() == false && mSendMessage->IsChildPending() == false)
     {
         mSendQueue.Dequeue(*mSendMessage);