DO NOT MERGE Update libpng to 1.6.20

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

check_error(display *d, png_uint_32 flags, const char *message)
{
 while (flags)
 {
      png_uint_32 flag = flags & -(png_int_32)flags;
 int i = find_by_flag(flag);

      fprintf(stderr, "%s(%s): chunk %s: %s\n", d->file, d->test,
         chunk_info[i].name, message);
 ++(d->error_count);

      flags &= ~flag;
 }
}

check_error(display *d, png_uint_32 flags, const char *message)
{
 while (flags)
 {
      png_uint_32 flag = flags & -(png_int_32)flags;
 int i = find_by_flag(flag);

      fprintf(stderr, "%s(%s): chunk %s: %s\n", d->file, d->test,
         chunk_info[i].name, message);
 ++(d->error_count);

      flags &= ~flag;
 }
}
