download: Actually validate empty pathnames

From the path module doc:
    If the path is empty, Base returns ".". If the path consists
    entirely of slashes, Base returns "/".

Actually validate that we provide a filename when one is required,
rather than failing with an error that /scratch exists but is not a
file.

Signed-off-by: Daniel Stone <[email protected]>

actions/download_action.go

@@ -76,7 +76,7 @@ func (d *DownloadAction) validateFilename(context *debos.DebosContext, url *url.
  } else {
  filename = path.Base(d.Filename)
  }
- if len(filename) == 0 {
+ if len(filename) == 0 || filename == "." || filename == "/" {
  return "", fmt.Errorf("Incorrect filename is provided for '%s'", d.Url)
  }
  filename = path.Join(context.Scratchdir, filename)