openvpn: more reneg-sec spots to adjust

(cherry picked from commit 48d4f752)

src/www/vpn_openvpn_export.php

@@ -629,7 +629,7 @@ function openvpn_client_export_sharedkey_config($srvid, $useaddr, $proxy, $zipco
     $conf .= "auth {$digest}\n";
     $conf .= "pull\n";
     $conf .= "resolv-retry infinite\n";
-    if (!empty($settings['reneg-sec'])) {
+    if (isset($settings['reneg-sec']) && $settings['reneg-sec'] != "") {
         $conf .= "reneg-sec {$settings['reneg-sec']}\n";
     }
     $conf .= "remote {$server_host} {$server_port}\n";

src/www/vpn_openvpn_server.php

@@ -318,7 +318,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
                 $input_errors[] = gettext("The Server Bridge DHCP range is invalid (start higher than end).");
             }
         }
-        if (!empty($pconfig['reneg-sec']) && (string)((int)$pconfig['reneg-sec']) != $pconfig['reneg-sec']) {
+        if (isset($pconfig['reneg-sec']) && (string)((int)$pconfig['reneg-sec']) != $pconfig['reneg-sec']) {
             $input_errors[] = gettext("Renegotiate time should contain a valid number of seconds.");
         }
         do_input_validation($pconfig, $reqdfields, $reqdfieldsn, $input_errors);