make valgrind a bit happier

readsb.c

@@ -457,6 +457,8 @@ static void cleanup_and_exit(int code) {
  while (s) {
  ns = s->next;
  free(s->listener_fds);
+ if (s->writer && s->writer->data)
+ free(s->writer->data);
  if (s) free(s);
  s = ns;
  }