message: eliminate MSVS2017 build warning
  modules\dnn\src\layers\prior_box_layer.cpp(208): warning C4834: discarding return value of function with 'nodiscard' attribute
diff: @@ -205,7 +205,9 @@ public:
  if (_explicitSizes)
  {
  - CV_Assert(_aspectRatios.empty(), !params.has("min_size"), !params.has("max_size"));
  + CV_Assert(_aspectRatios.empty());
  + CV_Assert(!params.has("min_size"));
  + CV_Assert(!params.has("max_size"));
  _boxWidths = widths;
  _boxHeights = heights;
  }
  