update init val. test=develop

lite/api/cxx_api.h

@@ -256,8 +256,8 @@ class CxxPaddleApiImpl : public lite_api::PaddlePredictor {
   bool status_is_cloned_;
 #ifdef LITE_WITH_CUDA
   bool multi_stream_{false};
-  cudaStream_t* io_stream_;
-  cudaStream_t* exec_stream_;
+  cudaStream_t* io_stream_{nullptr};
+  cudaStream_t* exec_stream_{nullptr};
   cudaEvent_t input_event_;
   std::vector<cudaEvent_t> output_events_;
   // only for multi exec stream mode.