Fixed race condition with two calls to port.once

The content script checks twice for which trackers are blocked, since the extension might not yet know about all blocked trackers on the first check (but we want the first check to happen as quickly as possible). These two checks sometimes interleave and cause an error (#162). So don't do the second check until the first has completed.
Hmm really? It shouldn't get here if head is null. I can add another check here just in case.