Fix display on X of 1-pixel R2L characters.

 src/xterm.c (x_draw_hollow_cursor): Fix display of hollow cursor on
 1-pixel R2L characters.
 Reported by Dmitry Antipov <dmantipov@yandex.ru>, see
 http://lists.gnu.org/archive/html/emacs-devel/2014-10/msg00518.html.