Fix -Werror=sign-compare
Fix -Werror=sign-compare

This should fix the fits thrown with all errors on.
OpenImageIO/oiio,bdeluca/oiio,scott-wilson/oiio,mcanthony/oiio,bdeluca/oiio,lgritz/oiio,lgritz/oiio,sambler/oiio,sambler/oiio,mcanthony/oiio,OpenImageIO/oiio,OpenImageIO/oiio,YangYangTL/oiio,micler/oiio,mcanthony/oiio,bdeluca/oiio,OpenImageIO/oiio,scott-wilson/oiio,bdeluca/oiio,YangYangTL/oiio,lgritz/oiio,micler/oiio,sambler/oiio,cwilling/oiio,YangYangTL/oiio,mcanthony/oiio,scott-wilson/oiio,sambler/oiio,cwilling/oiio,YangYangTL/oiio,cwilling/oiio,micler/oiio,micler/oiio,lgritz/oiio,cwilling/oiio,scott-wilson/oiio