[NETFILTER]: nfnetlink_log: micro-optimization for inst==NULL in nfulnl_recv_config()

Simple micro-optimization: don't call instance_put() on known NULL pointers.

Signed-off-by: Michal Miroslaw <mirq-linux@rere.qmqm.pl>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>