libavfilter/dnn/dnn_backend_tf: fix typo that variable uninitialized.

if it is initialized randomly, the tensorflow lib will report
error message such as:
Attempt to add output -7920 of depth_to_space4 not in range [0, 1) to node with type Identity
Signed-off-by: Guo, Yejun <yejun.guo@intel.com>
Signed-off-by: Pedro Arthur <bygrandao@gmail.com>

libavfilter/dnn/dnn_backend_tf.c

@@ -490,6 +490,7 @@ static DNNReturnType load_native_model(TFModel *tf_model, const char *model_file
 
     op_desc = TF_NewOperation(tf_model->graph, "Identity", "y");
     input.oper = op;
+    input.index = 0;
     TF_AddInput(op_desc, input);
     TF_FinishOperation(op_desc, tf_model->status);
     if (TF_GetCode(tf_model->status) != TF_OK){