Test that sk != NULL before reading sk->sk_tsflags.

Fixes: 49ca0d8b ("net-timestamp: no-payload option")
Reported-by: NOne Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>