ospf6d: Fix crash when '[no] ipv6 ospf6 advertise prefix-list' is in startup-config

* ospf6_interface.c: When '[no] ipv6 ospf6 advertise prefix-list'
  appears in the startup configuration for ospf6d, a crash occurs,
  because ospf6d attempts to schedule LSAs when the 'oi->area'
  structure has not yet been initialized.

  Now, when the command above is issued (either in the startup
  configuration or at runtime), ospf6d will only schedule LSAs if
  the 'oi->area' structure has been initalized. A similar test is
  already used when handling the commands 'ipv6 ospf6 priority'
  and 'ipv6 ospf6 cost'.
diff --git a/ospf6d/ospf6_interface.c b/ospf6d/ospf6_interface.c
index 777bc7c..cb34745 100644
--- a/ospf6d/ospf6_interface.c
+++ b/ospf6d/ospf6_interface.c
@@ -1394,13 +1394,17 @@
   oi->plist_name = XSTRDUP (MTYPE_PREFIX_LIST_STR, argv[0]);
 
   ospf6_interface_connected_route_update (oi->interface);
-  OSPF6_LINK_LSA_SCHEDULE (oi);
-  if (oi->state == OSPF6_INTERFACE_DR)
+
+  if (oi->area)
     {
-      OSPF6_NETWORK_LSA_SCHEDULE (oi);
-      OSPF6_INTRA_PREFIX_LSA_SCHEDULE_TRANSIT (oi);
+      OSPF6_LINK_LSA_SCHEDULE (oi);
+      if (oi->state == OSPF6_INTERFACE_DR)
+        {
+          OSPF6_NETWORK_LSA_SCHEDULE (oi);
+          OSPF6_INTRA_PREFIX_LSA_SCHEDULE_TRANSIT (oi);
+        }
+      OSPF6_INTRA_PREFIX_LSA_SCHEDULE_STUB (oi->area);
     }
-  OSPF6_INTRA_PREFIX_LSA_SCHEDULE_STUB (oi->area);
 
   return CMD_SUCCESS;
 }
@@ -1433,13 +1437,17 @@
     }
 
   ospf6_interface_connected_route_update (oi->interface);
-  OSPF6_LINK_LSA_SCHEDULE (oi);
-  if (oi->state == OSPF6_INTERFACE_DR)
+
+  if (oi->area)
     {
-      OSPF6_NETWORK_LSA_SCHEDULE (oi);
-      OSPF6_INTRA_PREFIX_LSA_SCHEDULE_TRANSIT (oi);
+      OSPF6_LINK_LSA_SCHEDULE (oi);
+      if (oi->state == OSPF6_INTERFACE_DR)
+        {
+          OSPF6_NETWORK_LSA_SCHEDULE (oi);
+          OSPF6_INTRA_PREFIX_LSA_SCHEDULE_TRANSIT (oi);
+        }
+      OSPF6_INTRA_PREFIX_LSA_SCHEDULE_STUB (oi->area);
     }
-  OSPF6_INTRA_PREFIX_LSA_SCHEDULE_STUB (oi->area);
 
   return CMD_SUCCESS;
 }