freetype-go: Fix panic when drawing a span to the right of the

bounding rect.

This bug was introduced when an image.RGBA's representation went from
slice of slices to one linear buffer.

R=r
CC=golang-dev
http://codereview.appspot.com/2020044

freetype/raster/paint.go

@@ -105,6 +105,9 @@ func (r *RGBAPainter) Paint(ss []Span, done bool) {
 		if s.X1 > b.Max.X {
 			s.X1 = b.Max.X
 		}
+		if s.X0 >= s.X1 {
+			continue
+		}
 		base := s.Y * r.Image.Stride
 		p := r.Image.Pix[base+s.X0 : base+s.X1]
 		for i, rgba := range p {