macsec: add missing NULL check after kmalloc

Fixes: c09440f7 ("macsec: introduce IEEE 802.1AE driver")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
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>