bug#4094: 23.1.50; posn-at-point gets upset by overlays with 'invisible and 'display

[Chong Yidong]

> posn-at-point reports bad (X . Y) when on a 'displa part of an invisible
> overlay.
>
> This makes line-move-visual behave strange. (A workaround for this could
> be to step back one char in line-move-visual, but this is of course not
> the correct fix.)

Please provide a precise, step-by-step recipe to reproduce this,
starting with `emacs -Q'.