plugins: protect calls to functions as done elsewhere

(cherry picked from commit baa813eb)

src/etc/rc.filter_synchronize

@@ -294,8 +294,11 @@ if (isset($config['hasync']) && is_array($config['hasync'])) {
     $section_cnf['synchronizeschedules'] = 'schedules';
     $section_cnf['synchronizeshaper'] = 'OPNsense.TrafficShaper';
     $section_cnf['synchronizecaptiveportal'] = 'OPNsense.captiveportal';
-    foreach (plugins_xmlrpc_sync() as $syncid => $syncconf) {
-        $section_cnf['synchronize'.$syncid] = $syncconf['section'];
+
+    if (function_exists('plugins_xmlrpc_sync')) {
+        foreach (plugins_xmlrpc_sync() as $syncid => $syncconf) {
+            $section_cnf['synchronize'.$syncid] = $syncconf['section'];
+        }
     }
 
     $sections = array();

src/www/system_hasync.php

@@ -41,7 +41,14 @@ $checkbox_names = array('pfsyncenabled', 'synchronizeusers', 'synchronizeauthser
                         'synchronizestaticroutes', 'synchronizelb', 'synchronizevirtualip',
                         'synchronizednsforwarder','synchronizednsresolver', 'synchronizeshaper', 'synchronizecaptiveportal'
 );
-foreach (array_keys(plugins_xmlrpc_sync()) as $key) {
+
+$syncplugins = array();
+
+if (function_exists('plugins_xmlrpc_sync')) {
+    $syncplugins = plugins_xmlrpc_sync();
+}
+
+foreach (array_keys($syncplugins) as $key) {
     $checkbox_names[] = 'synchronize'.$key;
 }
 
@@ -359,7 +366,7 @@ include("head.inc");
                 </tr>
                 <!-- Hook xmlrpc sync plugins -->
 <?php
-                foreach (plugins_xmlrpc_sync() as $syncid => $synccnf):?>
+                foreach ($syncplugins as $syncid => $synccnf):?>
                 <tr>
                   <td><a id="help_for_synchronize<?=$syncid?>" href="#" class="showhelp"><i class="fa fa-info-circle"></i></a> <?=$synccnf['description'];?></td>
                   <td>