Fix vertical cursor motion when there are overlay strings at EOL.

Fix vertical cursor motion when there are overlay strings at EOL.

 src/indent.c (Fvertical_motion): Don't consider display strings on
 overlay strings as display strings on the buffer position we
 started from.  This prevents vertical cursor motion from jumping
 more than one line when there's an overlay string with a display
 property at end of line.
 Reported by Karl Chen <Karl.Chen@quarl.org> in
 http://lists.gnu.org/archive/html/emacs-devel/2013-04/msg00362.html.