ospfd: Change struct ospf_path *oi to ifindex.

* global: In struct ospf_path, change struct ospf_interface *oi to int
  ifindex.  It is unsafe to reference *oi as an ospf interface can be
  deleted under your feet. Use a weak reference instead.
diff --git a/ospfd/ospf_ase.c b/ospfd/ospf_ase.c
index 044f97c..7bddf3f 100644
--- a/ospfd/ospf_ase.c
+++ b/ospfd/ospf_ase.c
@@ -135,6 +135,7 @@
 {
   struct listnode *node;
   struct ospf_path *op;
+  struct interface *ifp;
 
   for (ALL_LIST_ELEMENTS_RO (ro->paths, node, op))
     if (op->nexthop.s_addr == 0)
@@ -593,7 +594,7 @@
        
        if (! IPV4_ADDR_SAME (&op->nexthop, &newop->nexthop))
 	 return 0;
-       if (op->oi->ifp->ifindex != newop->oi->ifp->ifindex)
+       if (op->ifindex != newop->ifindex)
 	 return 0;
      }
    return 1;