fixed memory leaks in cvtyuv tests

(cherry picked from commit b2489d31d6)

modules/imgproc/test/test_cvtyuv.cpp

@@ -548,7 +548,7 @@ void referenceRGB2YUV(const Mat& rgb, Mat& yuv, RGBreader* rgbReader, YUVwriter*
 
 struct ConversionYUV
 {
-    ConversionYUV( const int code )
+    explicit ConversionYUV( const int code )
     {
         yuvReader_  = YUVreader :: getReader(code);
         yuvWriter_  = YUVwriter :: getWriter(code);
@@ -557,6 +557,24 @@ struct ConversionYUV
         grayWriter_ = GRAYwriter:: getWriter(code);
     }
 
+    ~ConversionYUV()
+    {
+        if (yuvReader_)
+            delete yuvReader_;
+
+        if (yuvWriter_)
+            delete yuvWriter_;
+
+        if (rgbReader_)
+            delete rgbReader_;
+
+        if (rgbReader_)
+            delete rgbReader_;
+
+        if (grayWriter_)
+            delete grayWriter_;
+    }
+
     int getDcn()
     {
         return (rgbWriter_ != 0) ? rgbWriter_->channels() : ((grayWriter_ != 0) ? grayWriter_->channels() : yuvWriter_->channels());