libpng has merged a patch from Chromium and included it in the v1.6.39 release, making PNG's over the PNG_USER_CHUNK_MALLOC_MAX limit only a benign error, meaning the image can still be displayed despite excessively large metadata chunks: pnggroup/libpng@33de006

The solution is sketchy, "Though it means we do allow libpng to allocate more than PNG_USER_CHUNK_MALLOC_MAX", a better solution should be chosen for spng.