ospfd: remove unneeded memset from a very hot function

* ospf_lsdb.c: (lsdb_prefix_set) memset is unneeded, as all fields are
initialised explicitly, and this function can be in the top-3 of a
profile when there are a lot of LSAs.
diff --git a/ospfd/ospf_lsdb.c b/ospfd/ospf_lsdb.c
index 2e1bded..c906f05 100644
--- a/ospfd/ospf_lsdb.c
+++ b/ospfd/ospf_lsdb.c
@@ -75,7 +75,6 @@
 static void
 lsdb_prefix_set (struct prefix_ls *lp, struct ospf_lsa *lsa)
 {
-  memset (lp, 0, sizeof (struct prefix_ls));
   lp->family = 0;
   lp->prefixlen = 64;
   lp->id = lsa->data->id;