better icc profile write for vips2png
better icc profile write for vips2png

tests for icc profile presence before reading, to avoid polluting the
error log

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