DO NOT MERGE Update libpng to 1.6.20

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

 buffer_extend(struct buffer_list *current)
 {
 struct buffer_list *add;

   assert(current->next == NULL);

   add = NEW(struct buffer_list);
 if (add == NULL)
 return NULL;

   add->next = NULL;
   current->next = add;

 return add;
}

 buffer_extend(struct buffer_list *current)
 {
 struct buffer_list *add;

   assert(current->next == NULL);

   add = NEW(struct buffer_list);
 if (add == NULL)
 return NULL;

   add->next = NULL;
   current->next = add;

 return add;
}
