fix: use `mb_substr`

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

lib/private/Repair/Owncloud/SaveAccountsTableData.php

@@ -174,7 +174,7 @@ class SaveAccountsTableData implements IRepairStep {
 
 		if ($userdata['display_name'] !== null) {
 			// user.displayname only allows 64 characters but old accounts.display_name allowed 255 characters
-			$update->setParameter('displayname', substr($userdata['display_name'], 0, 64))
+			$update->setParameter('displayname', mb_substr($userdata['display_name'], 0, 64))
 				->setParameter('userid', $userdata['user_id']);
 			$update->executeStatement();
 		}