chg: check lockfd at freecmdserveropts

We check for sockdirfd at freecmdserveropts but not lockfd, which is a bit
strange to people new to the code. Add a comment and an assert to make it
clear that lockfd should be closed earlier.

contrib/chg/chg.c

@@ -49,6 +49,7 b' static void freecmdserveropts(struct cmd'
 	free(opts->args);
 	opts->args = NULL;
 	opts->argsize = 0;
+	assert(opts->lockfd == -1 && "should be closed by unlockcmdserver()");
 	if (opts->sockdirfd != AT_FDCWD) {
 		close(opts->sockdirfd);
 		opts->sockdirfd = AT_FDCWD;