[NETFILTER]: nfnetlink_log: fix possible NULL pointer dereference

Eliminate possible NULL pointer dereference in nfulnl_recv_config().

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>