Fix cursor drawing in hscrolled R2L screen lines.

Fix cursor drawing in hscrolled R2L screen lines.

 src/xdisp.c (get_glyph_string_clip_rects): Don't let the width of a
 clipping rectangle become negative (i.e. large positive, since
 it's an unsigned data type).  This can happen in R2L hscrolled
 glyph rows, and caused us to draw the cursor glyph on the fringe.
 For the details, see
 http://lists.gnu.org/archive/html/emacs-devel/2014-08/msg00543.html.