Prevent alert when /dev/urandom is not accessible

Exception:

> Warning: file_exists() [<a href='function.file-exists'>function.file-exists</a>]: open_basedir restriction in effect. File(/dev/urandom) is not within the allowed path(s): (/usr/lib/php:/usr/local/lib/php:/tmp) in /.../vendor/friendsofsymfony/oauth2-php/lib/OAuth2/OAuth2.php line 1190