Added some assertions

src/gfx/generated_image.cpp

@@ -84,6 +84,7 @@ Image BlankImage::generate(const Options& opt) const {
   int w = max(1, opt.width >= 0  ? opt.width  : opt.height);
   int h = max(1, opt.height >= 0 ? opt.height : opt.width);
   Image img(w, h);
+  assert(img.Ok());
   img.InitAlpha();
   memset(img.GetAlpha(), 0, w * h);
   return img;

src/gfx/mask_image.cpp

@@ -134,6 +134,7 @@ void AlphaMask::convexHull(vector<wxPoint>& points) const {
 
 Image AlphaMask::colorImage(const Color& color) const {
   Image image(size.x, size.y);
+  assert(image.Ok());
   fill_image(image, color);
   setAlpha(image);
   return image;