`check_existence` flag incorrectly handled in filelistdatabase query

In https://gitlab.idiap.ch/bob/bob.bio.base/-/blob/3efccd3b637ee73ec68ed0ac5fde2667a943bd6e/bob/bio/base/database/filelist/query.py#L833, the check_existence flag is said to be ignored when multiple original extensions are specified, while it is actually not ignored.

Also, when only a single extension is specified, the check_existence flag is tested incorrectly.