netfilter: nft_rbtree: ignore inactive matching element with no descendants

If we find a matching element that is inactive with no descendants, we
jump to the found label, then crash because of nul-dereference on the
left branch.

Fix this by checking that the element is active and not an interval end
and skipping the logic that only applies to the tree iteration.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Tested-by: Anders K. Pedersen <akp@akp.dk>