MDL-82328 auth_oauth2: don't load custom profile field data too early.

By prematurely loading custom profile data in the constructor, under
specific circumstances it would happen before the page was fully
initialised which meant that any attempt at applying filters when
formatting profile field data would result in thrown exception.

auth/oauth2/classes/auth.php

@@ -63,7 +63,6 @@ class auth extends \auth_plugin_base {
     public function __construct() {
         $this->authtype = 'oauth2';
         $this->config = get_config('auth_oauth2');
-        $this->customfields = $this->get_custom_user_profile_fields();
     }
 
     /**
@@ -310,7 +309,7 @@ class auth extends \auth_plugin_base {
             return $userdata;
         }
 
-        $allfields = array_merge($this->userfields, $this->customfields);
+        $allfields = array_merge($this->userfields, $this->get_custom_user_profile_fields());
 
         // Go through each field from the external data.
         foreach ($externaldata as $fieldname => $value) {