macsec: add missing NULL check after kmalloc

macsec: add missing NULL check after kmalloc

Fixes: c09440f7dcb3 ("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>
(cherry picked from commit 72f2a05b8f367ee0d75584a6fbec7dbe7c144f27)

Orabug: 24614549

Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>