DO NOT MERGE Update libpng to 1.6.20

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

main(void)
{
 /* Exit code 0 on success. */
 return !read_png(stdin);
}

main(void)
{
 /* Exit code 0 on success. */
 return !read_png(stdin);
}
