bug#18417: 24.3.93; posn-at-point confused by fill-column-indicator

[Dmitry]

1. Download, put inside load-path and require fill-column-indicator.el,
from https://github.com/alpaker/Fill-Column-Indicator.
2. Open an empty buffer, add a newline inside it.
3. Toggle fcm-mode on. Notice the indicator at the end of the empty
line.
4. With point at the beginning of that line, evaluate

(posn-col-row (posn-at-point (point)))

and see it evaluate to (71 . 0).

The problem is probably not in posn-col-row, because (posn-x-y
(posn-at-point (point))) also returns a large non-zero value in car.

In GNU Emacs 24.3.93.3 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2014-09-05 on axl
Repository revision: 117482 
monnier@iro.umontreal.ca-20140904161426-2072ebabqpyhaadw
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:     Ubuntu 14.04.1 LTS