[NETFILTER]: nfnetlink_queue: fix possible NULL-ptr dereference

Fix NULL-ptr dereference when a config message for a non-existant
queue containing only an NFQA_CFG_PARAMS attribute is received.

Coverity #433

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