better behaviour for malformed PNGs
better behaviour for malformed PNGs

thanks Nicolas

lovell/libvips,wishabi/libvips,leiyangyou/libvips,mcanthony/libvips,wishabi/libvips,leiyangyou/libvips,mcanthony/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,wishabi/libvips,lovell/libvips,lovell/libvips,leiyangyou/libvips,leiyangyou/libvips,wishabi/libvips,mcanthony/libvips,mcanthony/libvips