[NETFILTER]: nf_queue: check if rerouter is present before using it

Every rerouter needs to provide a save and a reroute function, we don't
need to check for them. But we do need to check if a rerouter is registered
at all for the current family, with bridging for example packets of
unregistered families can hit nf_queue.

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