Return default interval if ra_param is not received for an interface

Change-Id: I4a8552d28ce09ff554dbff1750dda9f373b31bba
diff --git a/src/radv.c b/src/radv.c
index 1099e71..13e0afc 100644
--- a/src/radv.c
+++ b/src/radv.c
@@ -1035,6 +1035,7 @@
 static unsigned int calc_interval(struct ra_interface *ra)
 {
   int interval = DEFAULT_RTR_ADV_INTERVAL;
+  if (ra) {
   unsigned int min_rtr_interval = ra->min_ra_adv_interval;
   unsigned int max_rtr_interval = ra->max_ra_adv_interval;
 
@@ -1051,6 +1052,7 @@
   if (((ra->ra_sent_count + 1) < MAX_INITIAL_RTR_ADVERTISEMENTS) &&
        interval > MAX_INITIAL_RTR_ADVERT_INTERVAL)
     interval = random_num_in_range(MAX_INITIAL_RTR_ADVERT_INTERVAL);
+  }
   return (unsigned int)interval;
 }