Avoid division by 0 in the border rendering code

We skip sides with 0 border width in render_border, but when
we collect sides with the same style, we may pass the 0 width
down to render_frame_stroke anyway. So skip width 0 sides
there as well.

gtk/gtkrenderborder.c

@@ -523,6 +523,9 @@ render_frame_stroke (cairo_t       *cr,
           if (hidden_side & (1 << i))
             continue;
 
+          if (border_width[i] == 0)
+            continue;
+
           cairo_save (cr);
 
           if (i == 0)