DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)

transform_end(png_structp ppIn, png_infop pi)
{
   png_const_structp pp = ppIn;
   transform_display *dp = voidcast(transform_display*,
      png_get_progressive_ptr(pp));

 if (!dp->this.speed)
      transform_image_validate(dp, pp, pi);
 else
      dp->this.ps->validated = 1;
}

transform_end(png_structp ppIn, png_infop pi)
{
   png_const_structp pp = ppIn;
   transform_display *dp = voidcast(transform_display*,
      png_get_progressive_ptr(pp));

 if (!dp->this.speed)
      transform_image_validate(dp, pp, pi);
 else
      dp->this.ps->validated = 1;
}
