fix a crash in gifload with a missing colormap
fix a crash in gifload with a missing colormap

it seems gifs can have a missing colormap, who knew

thanks Kleis

see https://github.com/jcupitt/libvips/issues/773

lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips