Fix missing check that access_token and secret are set

In views/index.php, you check that both $settings[‘access_token’] and
$settings[‘access_token_secret’] are set, otherwise setting them to 
false. This check wasn’t being made in mod.twitter.php, which was 
causing `Undefined index: access token` and `Undefined index: access 
token_secret` errors for me.

This commit defaults those settings to false to avoid this error,
as was already done in views/index.php