fix signed / unsigned comparison error
fix signed / unsigned comparison error

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