ospf6d: CVE-2011-3323 (fortify packet reception)

This vulnerability (CERT-FI #514840) was reported by CROSS project.

ospf6d processes IPv6 prefix structures in incoming packets without
verifying that the declared prefix length is valid. This leads to a
crash
caused by out of bounds memory access.

* ospf6_abr.h: new macros for size/alignment validation
* ospf6_asbr.h: idem
* ospf6_intra.h: idem
* ospf6_lsa.h: idem
* ospf6_message.h: idem
* ospf6_proto.h: idem
* ospf6_message.c
  * ospf6_packet_minlen: helper array for ospf6_packet_examin()
  * ospf6_lsa_minlen: helper array for ospf6_lsa_examin()
  * ospf6_hello_recv(): do not call ospf6_header_examin(), let upper
    layer verify the input data
  * ospf6_dbdesc_recv(): idem
  * ospf6_lsreq_recv(): idem
  * ospf6_lsupdate_recv(): idem
  * ospf6_lsack_recv(): idem
  * ospf6_prefixes_examin(): new function, implements A.4.1
  * ospf6_lsa_examin(): new function, implements A.4
  * ospf6_lsaseq_examin(): new function, an interface to above
  * ospf6_packet_examin(): new function, implements A.3
  * ospf6_rxpacket_examin(): new function, replaces
    ospf6_header_examin()
  * ospf6_header_examin(): sayonara
  * ospf6_receive(): perform passive interface check earliest possible,
    employ ospf6_rxpacket_examin()
diff --git a/ospf6d/ospf6_intra.h b/ospf6d/ospf6_intra.h
index 31643fd..3810174 100644
--- a/ospf6d/ospf6_intra.h
+++ b/ospf6d/ospf6_intra.h
@@ -69,6 +69,7 @@
    conf_debug_ospf6_brouter_specific_area_id == (area_id))
 
 /* Router-LSA */
+#define OSPF6_ROUTER_LSA_MIN_SIZE              4U
 struct ospf6_router_lsa
 {
   u_char bits;
@@ -77,6 +78,7 @@
 };
 
 /* Link State Description in Router-LSA */
+#define OSPF6_ROUTER_LSDESC_FIX_SIZE          16U
 struct ospf6_router_lsdesc
 {
   u_char    type;
@@ -105,6 +107,7 @@
   (((struct ospf6_router_lsdesc *)(x))->neighbor_router_id)
 
 /* Network-LSA */
+#define OSPF6_NETWORK_LSA_MIN_SIZE             4U
 struct ospf6_network_lsa
 {
   u_char reserved;
@@ -113,6 +116,7 @@
 };
 
 /* Link State Description in Router-LSA */
+#define OSPF6_NETWORK_LSDESC_FIX_SIZE          4U
 struct ospf6_network_lsdesc
 {
   u_int32_t router_id;
@@ -121,6 +125,7 @@
   (((struct ospf6_network_lsdesc *)(x))->router_id)
 
 /* Link-LSA */
+#define OSPF6_LINK_LSA_MIN_SIZE               24U /* w/o 1st IPv6 prefix */
 struct ospf6_link_lsa
 {
   u_char          priority;
@@ -131,6 +136,7 @@
 };
 
 /* Intra-Area-Prefix-LSA */
+#define OSPF6_INTRA_PREFIX_LSA_MIN_SIZE       12U /* w/o 1st IPv6 prefix */
 struct ospf6_intra_prefix_lsa
 {
   u_int16_t prefix_num;