Fix unknown subtype handling in oval_subtype_parse
Fix unknown subtype handling in oval_subtype_parse

See #986 discussion
OpenSCAP/openscap,redhatrises/openscap,mpreisler/openscap,mpreisler/openscap,mpreisler/openscap,OpenSCAP/openscap,OpenSCAP/openscap,redhatrises/openscap,jan-cerny/openscap,jan-cerny/openscap,mpreisler/openscap,redhatrises/openscap,mpreisler/openscap,jan-cerny/openscap,redhatrises/openscap,OpenSCAP/openscap,jan-cerny/openscap,OpenSCAP/openscap,jan-cerny/openscap,mpreisler/openscap,jan-cerny/openscap,redhatrises/openscap,redhatrises/openscap,OpenSCAP/openscap