[NETFILTER]: nfnetlink_log: fix checks in nfulnl_recv_config

Similar to the nfnetlink_queue fixes:

The peer_pid must be checked in all cases when a logging instance exists,
additionally we must check whether an instance exists before attempting
to configure it to avoid NULL ptr dereferences.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>