message: Removed Warning in the lstm.cc file
  Removed 3 warning from the lstm.cc file
diff: @@ -841,6 +841,7 @@ void* Init(TfLiteContext* context, const char* buffer, size_t length) {
  case kTfLiteLSTMBasicKernel:
  return basic::Init(context, buffer, length);
  }
  + return nullptr;
  }
  void Free(TfLiteContext* context, void* buffer) {
  gemm_support::DecrementUsageCounter(context);
  @@ -856,6 +857,7 @@ TfLiteStatus Prepare(TfLiteContext* context, TfLiteNode* node) {
  case kTfLiteLSTMBasicKernel:
  return basic::Prepare(context, node);
  }
  + return kTfLiteError;
  }
  TfLiteStatus Eval(TfLiteContext* context, TfLiteNode* node) {
  @@ -866,6 +868,7 @@ TfLiteStatus Eval(TfLiteContext* context, TfLiteNode* node) {
  case kTfLiteLSTMBasicKernel:
  return basic::Eval(context, node);
  }
  + return kTfLiteError;
  }
  } // namespace lstm
  