[tcp] Fix potential NULL pointer dereference

Detected using Valgrind.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp.c

 	flags = tcphdr->flags;
 	tcp_rx_opts ( tcp, ( ( ( void * ) tcphdr ) + sizeof ( *tcphdr ) ),
 		      ( hlen - sizeof ( *tcphdr ) ), &options );
-	if ( options.tsopt )
+	if ( tcp && options.tsopt )
 		tcp->ts_val = ntohl ( options.tsopt->tsval );
 	iob_pull ( iobuf, hlen );
 	len = iob_len ( iobuf );