partially fix limit total number of preview links

Old method, if follows exceeded limit, then extension would throw an
error as filtered doesn’t exist in the else clause.

To test: change limit to something like 1 and it'll only show one channel (instead of however many you have followed and are online. It doesn't filter the number of API calls though at the moment (but at least the extension won't break now).

A better (and more complex solution) is to pass the limit the channel parser, which only returns the callback once either we've hit all channels checked (which is what it does now) or it hits a specific limit. Haven't looked at the async call enough to properly insert that limit right now.