DO NOT MERGE Update libpng to 1.6.20

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

store_freenew(png_store *ps)
{
   store_freebuffer(&ps->new);
   ps->writepos = 0;
 if (ps->palette != NULL)
 {
      free(ps->palette);
      ps->palette = NULL;
      ps->npalette = 0;
 }
}

store_freenew(png_store *ps)
{
   store_freebuffer(&ps->new);
   ps->writepos = 0;
 if (ps->palette != NULL)
 {
      free(ps->palette);
      ps->palette = NULL;
      ps->npalette = 0;
 }
}
