BFD: Fix invalid read from pollfd array

It is possible that sockets_add() are called between sockets_prepare()
and sockets_fire() during poll loop in birdloop_main(), so we need to
use loop->poll_fd.used instead of loop->sock_num to find the last field.

proto/bfd/io.c

@@ -589,7 +589,7 @@ sockets_fire(struct birdloop *loop)
   times_update(loop);
 
   /* Last fd is internal wakeup fd */
-  if (pfd[loop->sock_num].revents & POLLIN)
+  if (pfd[poll_num].revents & POLLIN)
     wakeup_drain(loop);
 
   int i;