Update common.php

Minor change to check if the $_SESSION['lang'] is set, and avoid following errors :

> Notice: Undefined index: lang in codiad/common.php on line 10
> Warning: include(/codiad/languages/.php): failed to open stream: No such file or directory in codiad/common.php on line 10
> Warning: include(): Failed opening '/codiad/languages/.php' for inclusion (include_path='.:/usr/share/php:/usr/share/pear') in codiad/common.php on line 10