sock: fix possible NULL sk dereference in __skb_tstamp_tx

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

Fixes: 49ca0d8 ("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>

net/core/skbuff.c

@@ -3733,9 +3733,13 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
 		     struct sock *sk, int tstype)
 {
 	struct sk_buff *skb;
-	bool tsonly = sk->sk_tsflags & SOF_TIMESTAMPING_OPT_TSONLY;
+	bool tsonly;
 
-	if (!sk || !skb_may_tx_timestamp(sk, tsonly))
+	if (!sk)
+		return;
+
+	tsonly = sk->sk_tsflags & SOF_TIMESTAMPING_OPT_TSONLY;
+	if (!skb_may_tx_timestamp(sk, tsonly))
 		return;
 
 	if (tsonly)