[tcp] Avoid potential NULL pointer dereference

Commit ea61075 ("[tcp] Add support for TCP window scaling") introduced
a potential NULL pointer dereference by referring to the connection's
send window scale before checking whether or not the connection is
known.

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

src/net/tcp.c

 	uint16_t csum;
 	uint32_t seq;
 	uint32_t ack;
+	uint16_t raw_win;
 	uint32_t win;
 	unsigned int flags;
 	size_t len;
 	tcp = tcp_demux ( ntohs ( tcphdr->dest ) );
 	seq = ntohl ( tcphdr->seq );
 	ack = ntohl ( tcphdr->ack );
-	win = ( ntohs ( tcphdr->win ) << tcp->snd_win_scale );
+	raw_win = ntohs ( tcphdr->win );
 	flags = tcphdr->flags;
 	tcp_rx_opts ( tcp, ( ( ( void * ) tcphdr ) + sizeof ( *tcphdr ) ),
 		      ( hlen - sizeof ( *tcphdr ) ), &options );
 
 	/* Handle ACK, if present */
 	if ( flags & TCP_ACK ) {
+		win = ( raw_win << tcp->snd_win_scale );
 		if ( ( rc = tcp_rx_ack ( tcp, ack, win ) ) != 0 ) {
 			tcp_xmit_reset ( tcp, st_src, tcphdr );
 			goto discard;