DO NOT MERGE Update libpng to 1.6.20

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

display_clean_read(struct display *dp)
{
 if (dp->read_pp != NULL)
      png_destroy_read_struct(&dp->read_pp, &dp->read_ip, NULL);
}

display_clean_read(struct display *dp)
{
 if (dp->read_pp != NULL)
      png_destroy_read_struct(&dp->read_pp, &dp->read_ip, NULL);
}
