ospf6d: Fix LSA formatting out-of-bounds access

ospf6d: Fix LSA formatting out-of-bounds access

Check whether full struct ospf6_router_lsdesc/ospf6_prefix is accessible
before accessing its contents. Previously, we only checked for the first
byte in ospf6_router_lsa_get_nbr_id() or not even that (due to an additional
off-by-one error) in ospf6_link_lsa_get_prefix_str() and
ospf6_intra_prefix_lsa_get_prefix_str().

Also check *before* accessing the first prefix instead of starting the
checks only at the 2nd prefix.

The previous code could cause out-of-bounds accesses with valid LSAs in case
of ospf6_link_lsa_get_prefix_str() and
ospf6_intra_prefix_lsa_get_prefix_str() and with specially crafted LSAs
(bad length field) in case of ospf6_router_lsa_get_nbr_id().

Signed-off-by: Martin Buck <mb-tmp-tvguho.pbz@gromit.dyndns.org>