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_spf.c b/ospfd/ospf_spf.c
index 12afe5d..9566843 100644
--- a/ospfd/ospf_spf.c
+++ b/ospfd/ospf_spf.c
@@ -1077,13 +1077,14 @@
                 {
                   if (IS_DEBUG_OSPF_EVENT)
                     zlog_debug ("   directly attached to %s\r\n",
-                               IF_NAME (path->oi));
+				ifindex2ifname (path->ifindex));
                 }
               else
                 {
                   if (IS_DEBUG_OSPF_EVENT)
                     zlog_debug ("   via %s, %s\r\n",
-                               inet_ntoa (path->nexthop), IF_NAME (path->oi));
+				inet_ntoa (path->nexthop),
+				ifindex2ifname (path->ifindex));
                 }
             }
         }