Before upgrade glibc, install the libidn2 and libunistring first and then the rebuilded slapt-get package which use upgradepkg instead of spkg as back end of 'slapt-get --upgrade' to avoid that upgrading the package glibc fails, breaking your system. libidn2 is a dep for slapt-get so if you upgrade slapt-get first then it will not run until manually download and install libidn2 and libunistring from slackware current l group of packages repo.