[cvsnt] BUG: Update with non-existing tag deletes sandbox files

[jml at nykredit.dk]

   This bug applies to CVSNT version 2.5.04 build 3236.
   If a module is checked out or updated, with the -r option, (for
   example: CVS up -r this-tag-does-not-exist mymodule) and the tag does
   not exist (maybe because you missplelled it) CVS will report every
   file in the module as "no longer in the repository" and DELETE the
   file from the sandbox!
   Previous releases of CVSNT - correctly - reports the non-existing tag
   as, well - non-existing: "no such tag this-tag-does-not-exist".
   Best regards
   Jørgen Møller Larsen