test for res-unit meta before fetching
test for res-unit meta before fetching

if the res-unit meta is missing, don't generate an error message

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