message: Prevent null pointer dereference in decode_gif.
diff: @@ -140,6 +140,10 @@ uint8* Decode(const void* srcdata, int datasize,
  ColorMapObject* color_map = this_image->ImageDesc.ColorMap
  ? this_image->ImageDesc.ColorMap
  : gif_file->SColorMap;
  + if (color_map == nullptr) {
  + *error_string = strings::StrCat("missing color map for frame ", k);
  + return nullptr;
  + }
  for (int i = imgTop; i < imgBottom; ++i) {
  uint8* p_dst = this_dst + i * width * channel;
  