Panic can occur during http poller force

See https://issues.couchbase.com/browse/MB-44726 for more detail but we are somehow closing the stop channel twice in the cccp poller. From what I can see this can only happen if Stop is called before ForceHTTPPoller is triggered. We should probably set the active poller to nil in Stop and return early from ForceHTTPPoller if the active poller is nil.