MDL-60738 webservice: Clean theme and lang properly

Sometimes the "theme" and "lang" fields in the user and course tables
in the database are set to incorrect values (uninstalled or
non-existent themes and language packs).
This makes Web Services functions to fail because the WS server
validate the returned data using the validate_param function that clean
parameters.

user/lib.php

@@ -564,6 +564,14 @@ function user_get_user_details($user, $course = null, array $userfields = array(
         }
     }
 
+    // Clean lang and auth fields for external functions (it may content uninstalled themes or language packs).
+    if (isset($userdetails['lang'])) {
+        $userdetails['lang'] = clean_param($userdetails['lang'], PARAM_LANG);
+    }
+    if (isset($userdetails['theme'])) {
+        $userdetails['theme'] = clean_param($userdetails['theme'], PARAM_THEME);
+    }
+
     return $userdetails;
 }