ospf6d: clear DR info on interface_down

This fixes an issue where ospf6d would send incorrect hellos and
perform wrong DR election when an interface went down and up
again.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
diff --git a/ospf6d/ospf6_interface.c b/ospf6d/ospf6_interface.c
index c692f2c..86c0bf6 100644
--- a/ospf6d/ospf6_interface.c
+++ b/ospf6d/ospf6_interface.c
@@ -769,6 +769,10 @@
   
   list_delete_all_node (oi->neighbor_list);
 
+  /* When interface state is reset, also reset information about
+   * DR election, as it is no longer valid. */
+  oi->drouter = oi->prev_drouter = htonl(0);
+  oi->bdrouter = oi->prev_bdrouter = htonl(0);
   return 0;
 }