string_ref: make substr past end just return empty instead of exception
string_ref: make substr past end just return empty instead of exception

OpenImageIO/oiio,YangYangTL/oiio,scott-wilson/oiio,lgritz/oiio,lgritz/oiio,scott-wilson/oiio,cwilling/oiio,sambler/oiio,mcanthony/oiio,YangYangTL/oiio,micler/oiio,sambler/oiio,mcanthony/oiio,micler/oiio,cwilling/oiio,sambler/oiio,OpenImageIO/oiio,cwilling/oiio,OpenImageIO/oiio,mcanthony/oiio,scott-wilson/oiio,bdeluca/oiio,YangYangTL/oiio,bdeluca/oiio,lgritz/oiio,scott-wilson/oiio,bdeluca/oiio,cwilling/oiio,lgritz/oiio,bdeluca/oiio,OpenImageIO/oiio,sambler/oiio,mcanthony/oiio,micler/oiio,micler/oiio,YangYangTL/oiio