Fix compiler warning about signed/unsigned comparison
Fix compiler warning about signed/unsigned comparison

OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,micler/oiio,lgritz/oiio,YangYangTL/oiio,micler/oiio,OpenImageIO/oiio,YangYangTL/oiio,bdeluca/oiio,micler/oiio,lgritz/oiio,bdeluca/oiio,micler/oiio,YangYangTL/oiio,OpenImageIO/oiio,YangYangTL/oiio,OpenImageIO/oiio,bdeluca/oiio,bdeluca/oiio