Coverity: fixed some negative argument issues

modules/imgproc/src/drawing.cpp

@@ -2331,10 +2331,10 @@ static void addChildContour(InputArrayOfArrays contours,
 
         int h_next = hierarchy[i][0], h_prev = hierarchy[i][1],
             v_next = hierarchy[i][2], v_prev = hierarchy[i][3];
-        seq[i].h_next = (size_t)h_next < ncontours ? &seq[h_next] : 0;
-        seq[i].h_prev = (size_t)h_prev < ncontours ? &seq[h_prev] : 0;
-        seq[i].v_next = (size_t)v_next < ncontours ? &seq[v_next] : 0;
-        seq[i].v_prev = (size_t)v_prev < ncontours ? &seq[v_prev] : 0;
+        seq[i].h_next = (0 <= h_next && h_next < (int)ncontours) ? &seq[h_next] : 0;
+        seq[i].h_prev = (0 <= h_prev && h_prev < (int)ncontours) ? &seq[h_prev] : 0;
+        seq[i].v_next = (0 <= v_next && v_next < (int)ncontours) ? &seq[v_next] : 0;
+        seq[i].v_prev = (0 <= v_prev && v_prev < (int)ncontours) ? &seq[v_prev] : 0;
 
         if( v_next >= 0 )
             addChildContour(contours, ncontours, hierarchy, v_next, seq, block);

modules/photo/src/tonemap.cpp

@@ -510,8 +510,11 @@ protected:
 
     void calculateSum(std::vector<Mat>& x_contrast, std::vector<Mat>& y_contrast, Mat& sum)
     {
-        sum = Mat::zeros(x_contrast[x_contrast.size() - 1].size(), CV_32F);
-        for(int i = (int)x_contrast.size() - 1; i >= 0; i--)
+        if (x_contrast.empty())
+            return;
+        const int last = (int)x_contrast.size() - 1;
+        sum = Mat::zeros(x_contrast[last].size(), CV_32F);
+        for(int i = last; i >= 0; i--)
         {
             Mat grad_x, grad_y;
             getGradient(x_contrast[i], grad_x, 1);