macsec: don't put a NULL rxsa

The "deliver:" path of macsec_handle_frame can be called with
rx_sa == NULL.  Check rx_sa != NULL before calling macsec_rxsa_put().

Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macsec.c

@@ -1161,7 +1161,8 @@ deliver:
 			    macsec_extra_len(macsec_skb_cb(skb)->has_sci));
 	macsec_reset_skb(skb, secy->netdev);
 
-	macsec_rxsa_put(rx_sa);
+	if (rx_sa)
+		macsec_rxsa_put(rx_sa);
 	count_rx(dev, skb->len);
 
 	rcu_read_unlock();