dropbox: fix missing encoding for rclone purge

This was causing directories with encodable characters in not to be
found on purge.

See: https://forum.rclone.org/t/purge-command-does-not-work-on-directories-with-files/42793

backend/dropbox/dropbox.go

@@ -946,6 +946,7 @@ func (f *Fs) purgeCheck(ctx context.Context, dir string, check bool) (err error)
 	if root == "/" {
 		return errors.New("can't remove root directory")
 	}
+	root = f.opt.Enc.FromStandardPath(root)
 
 	if check {
 		// check directory exists
@@ -954,7 +955,6 @@ func (f *Fs) purgeCheck(ctx context.Context, dir string, check bool) (err error)
 			return fmt.Errorf("Rmdir: %w", err)
 		}
 
-		root = f.opt.Enc.FromStandardPath(root)
 		// check directory empty
 		arg := files.ListFolderArg{
 			Path:      root,