you should never play with glibc, libc and glib, at least on slackware and other low package distribution. However, that happen to everyone (that once happened to me) and it is possible to save your box, but that will be hard.
You should reinstall slackware glibc-2.1.3 with the old slackware 7.1 .tgz package.
Another (last chance) way to do is the following : get another harddrive, or something like this (any big enought device will be good I believe). Install a minimal old slackware 7.1 on it, then plug these two harddrive together... and then move everything that may have been altered from the new install to your old one.
It is almost impossible to upgrade glibc on slackware... in fact, slackware is very hard to upgrade at my opinion, I like it much for it flexibility, but soon you have to reinstall. If you really want to give a try, keep in mind that glibc has tons of dependency, you'll have to install glibc, glib, libc, ...
If you suceed, however, let me know