improve vipsthumbnail with broken images
improve vipsthumbnail with broken images

if there was an incorrect embedded profile, no fallback import profile,
and an export profile, vipsthumbnail could crash

see https://github.com/jcupitt/libvips/issues/186

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