photo: add assertion on empty image in denoising

modules/photo/src/denoising.cpp

@@ -116,6 +116,7 @@ void cv::fastNlMeansDenoising( InputArray _src, OutputArray _dst, const std::vec
     CV_INSTRUMENT_REGION()
 
     int hn = (int)h.size(), type = _src.type(), depth = CV_MAT_DEPTH(type), cn = CV_MAT_CN(type);
+    CV_Assert(!_src.empty());
     CV_Assert(hn == 1 || hn == cn);
 
     Size src_size = _src.size();