Fix for unsubscribe before subscribe scenario

[mocheng]

If mqtt client send unsubscribe message before any subscribe message. The (!sub || !sub.handler) expression would raise an exception.

[mcollina]

Can you please add a unit test for this? Thanks!

[mocheng]

Each subscriptions element has a "handler" field. The (!sub && !sub.handler) is same as (!sub). There is no way to tweak the client subscriptons field in UT.

[mcollina]

I agree. I think we might want to introduce a subscription object.
See https://github.com/mcollina/mosca/blob/master/lib/client.js#L402 how it is instantiated.

Something like:

function Subscription(qos, handler) {
  this.qos = qos
  this.handler = handler
}

[mcollina]

Any updates on this?

[mcollina]

Why closing?

[mocheng]

Oops. Seems I closed the wrong issue.

I recalled that I found this issue while unsubscribing a topic before any subscriptions. There should be a unit test case covering this scenario.

Anyway, mosca server should not crash on unexpected client behaviour.

I'll submit a pull request.