Expire Hello history immediately if interval is uninitialised.

This fixes the case where a neighbour only sends unscheduled Hellos.

neighbour.c

@@ -131,12 +131,13 @@ update_neighbour(struct neighbour *neigh, struct hello_history *hist,
     int rc = 0;
 
     if(hello < 0) {
-        if(hist->interval <= 0)
-            return rc;
+        if(hist->interval > 0)
             missed_hellos =
                 ((int)timeval_minus_msec(&now, &hist->time) -
                  hist->interval * 7) /
                 (hist->interval * 10);
+        else
+            missed_hellos = 16; /* infinity */
         if(missed_hellos <= 0)
             return rc;
         timeval_add_msec(&hist->time, &hist->time,