MDL-24530 Fixed core functions usage during early install phase when the site is not fully set up yet

lib/adminlib.php

@@ -2605,8 +2605,10 @@ class admin_setting_users_with_capability extends admin_setting_configmultiselec
             '$@NONE@$' => get_string('nobody'),
             '$@ALL@$' => get_string('everyonewhocan', 'admin', get_capability_string($this->capability)),
         );
-        foreach ($users as $user) {
-            $this->choices[$user->username] = fullname($user);
+        if (is_array($users)) {
+            foreach ($users as $user) {
+                $this->choices[$user->username] = fullname($user);
+            }
         }
         return true;
     }

lib/moodlelib.php

@@ -2683,7 +2683,6 @@ function ismoving($courseid) {
  * @param bool $override If true then the name will be first name followed by last name rather than adhering to fullnamedisplay setting.
  */
 function fullname($user, $override=false) {
-
     global $CFG, $SESSION;
 
     if (!isset($user->firstname) and !isset($user->lastname)) {
@@ -2703,7 +2702,7 @@ function fullname($user, $override=false) {
         $CFG->fullnamedisplay = $SESSION->fullnamedisplay;
     }
 
-    if ($CFG->fullnamedisplay == 'firstname lastname') {
+    if (!isset($CFG->fullnamedisplay) or $CFG->fullnamedisplay === 'firstname lastname') {
         return $user->firstname .' '. $user->lastname;
 
     } else if ($CFG->fullnamedisplay == 'lastname firstname') {