net/ncsi: check for null return from call to nla_nest_start

The call to nla_nest_start calls nla_put which can lead to a NULL
return so it's possible for attr to become NULL and we can potentially
get a NULL pointer dereference on attr.  Fix this by checking for
a NULL return.

Detected by CoverityScan, CID#1466125 ("Dereference null return")

Fixes: 955dc68c

 ("net/ncsi: Add generic netlink family")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>