Tensor.DisposeUnmanagedResources: Nullify _handle after delete.

src/TensorFlowNET.Core/Tensors/Tensor.cs

@@ -571,6 +571,7 @@ namespace Tensorflow
             if (handle != IntPtr.Zero)
             {
                 c_api.TF_DeleteTensor(handle);
+                _handle = IntPtr.Zero;
             }
         }