Fix issue with blind/batch uploaded file descriptions

When no file description was provided (e.g. blind/batch file uploads), I noticed that the short file description (summary) could be set to "(null)". This problem was introduced in commit bc7030d3 with the sbbsfile.des creation and post-processing support.

I also noticed that the extended description processing in sbbs_t::uploadfile() was not entirely consistent with other methods of adding/uploading files. Let prep_file_desc() do its just of processing the extended description into a suitable short description/summary (including truncation).