src/java.desktop/share/native/libfontmanager/scriptMapping.c has

hb_script_t getHBScriptCode(int code) {
    if (code > MAX_ICU_SCRIPTCODE) {
        return HB_SCRIPT_INVALID;
    }

Since 'code' will index an array, even though it is drawn from a limited set, it will be safer to ensure it is also >=0