Safety check for RA interface access

Change-Id: I4ad03e0da4ad3471c46fed02f50085e9481bb2d3
diff --git a/src/radv.c b/src/radv.c
index 1a79d99..67ee482 100644
--- a/src/radv.c
+++ b/src/radv.c
@@ -1037,7 +1037,7 @@
 static void new_timeout(struct dhcp_context *context, char *iface_name, time_t now)
 {
   struct ra_interface *ra = find_iface_param(iface_name);
-  if (ra->ra_sent_count < (MAX_INITIAL_RTR_ADVERTISEMENTS-1))
+  if (ra && ra->ra_sent_count < (MAX_INITIAL_RTR_ADVERTISEMENTS-1))
     context->ra_time = now + MAX_INITIAL_RTR_ADVERT_INTERVAL;
   else
     {