mount: fix panic on File.Open - Fixes #3595
mount: fix panic on File.Open - Fixes #3595

This problem was introduced in "mount: allow files of unkown size to
be read properly" 0baafb158f6dc5ab by failure to check that the
DirEntry was nil or not.

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone