fix memory crash bug by freeing m_scopeNetworkOperationMode before

EvalBase:Destroy(), since m_scopeNetworkOperationMode requires a reference
to the network.

Source/EvalDll/CNTKEval.cpp

@@ -438,6 +438,8 @@ void CNTKEvalExtended<ElemType>::ForwardPass(const ValueRefs<ElemType>& inputs,
 template <typename ElemType>
 void CNTKEvalExtended<ElemType>::Destroy()
 {
+    // Since m_scopeNetworkOperationMode has a reference to m_net, it has to be released first.
+    m_scopedNetworkOperationMode.reset();
     CNTKEvalBase<ElemType>::Destroy();
     delete this;
 }