ospf6d: Fix memory allocation issues in SPF

* ospf6_area.c: Call ospf6_spf_table_finish() before deleting the spf
    table.  This ensures that the associated ospf6_vertex structures
    are also freed.
* ospf6_spf.c: Only allocate a priority queue when a spf calculation
    is actually performed.
diff --git a/ospf6d/ospf6_area.c b/ospf6d/ospf6_area.c
index 0224fe6..9934e6b 100644
--- a/ospf6d/ospf6_area.c
+++ b/ospf6d/ospf6_area.c
@@ -199,6 +199,7 @@
   ospf6_lsdb_delete (oa->lsdb);
   ospf6_lsdb_delete (oa->lsdb_self);
 
+  ospf6_spf_table_finish (oa->spf_table);
   ospf6_route_table_delete (oa->spf_table);
   ospf6_route_table_delete (oa->route_table);