sock: fix possible NULL sk dereference in __skb_tstamp_tx

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

Fixes: 49ca0d8b

 ("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>