Test that sk != NULL before reading sk->sk_tsflags.
Fixes: 49ca0d8bfaf3 ("net-timestamp: no-payload option") Reported-by: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk> Signed-off-by: Willem de Bruijn <willemb@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>