Fix missing check that access_token and access_token_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 raising `Undefined index: access token` and `Undefined index: access  token_secret` for me when using the `{exp:twitter:user}` tag.

This commit defaults those settings to `FALSE` to avoid this error, as was already done in views/index.php