DO NOT MERGE Update libpng to 1.6.20

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

modification_reset(png_modification *pmm)
{
 if (pmm != NULL)
 {
      pmm->modified = 0;
      pmm->added = 0;
      pmm->removed = 0;
      modification_reset(pmm->next);
 }
}

modification_reset(png_modification *pmm)
{
 if (pmm != NULL)
 {
      pmm->modified = 0;
      pmm->added = 0;
      pmm->removed = 0;
      modification_reset(pmm->next);
 }
}
