[NETFILTER] nf_queue: Fix Ooops when no queue handler registered

With the new nf_queue generalization in 2.6.14, we've introduced a bug
that causes an oops as soon as a packet is queued but no queue handler
registered.  This patch fixes it.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>