netfilter: conntrack: check for NEXTHDR_NONE before header sanity checking

NEXTHDR_NONE doesn't has an IPv6 option header, so the first check
for the length will always fail and results in a confusing message
"too short" if debugging enabled. With this patch, we check for
NEXTHDR_NONE before length sanity checkings are done.

Signed-off-by: Christoph Paasch <christoph.paasch@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>