l2tp: reset conf_secret if secret removed from accel-ppp.conf

The "reload" command frees the variable pointed to by conf_secret
in case the "secret" option gets removed from accel-ppp.conf. In
such a case, conf_secret must be reset to NULL to avoid dereferencing
the old pointer.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>

accel-pppd/ctrl/l2tp/l2tp.c

@@ -3976,6 +3976,9 @@ static void load_config(void)
 	if (opt) {
 		conf_secret = opt;
 		conf_secret_len = strlen(opt);
+	} else {
+		conf_secret = NULL;
+		conf_secret_len = 0;
 	}
 
 	opt = conf_get_opt("l2tp", "dir300_quirk");