Fix skip non-semver tag names, instead of crashing

[schadomi7]

As discussed in discord: If a tag name is not a valid semver number, it gets cleaned to null. I propose to just skip offending tag names. Please check if it has other consequences, I don't know.

Alternatively, the check could be made more explicit:

     let curVer = semver.clean(opts.tags[i].name);
-    if (curVer && curVer !== LATEST_VER) {
+    if (curVer !== null && curVer !== LATEST_VER) {
       out.versions[curVer] = buildAbsentVer(curVer, opts);
     }