NPE fix for closing with pending AckClosure(s) on Android

[ambientlabs]

Calling publish on an instance of StreamingConnection with an AckHandler can result in NPEs if the connection is closed before ACKs have been sent. The result should be an "java.lang.IllegalStateException: stan: connection closed"; however, the result in an NPE.

Note: I am working on Android using Kotlin.

  val opts: Options? = Options.Builder().natsUrl("nats://10.0.2.2:4222").clientId("android-client").clusterId("test-cluster").build()
        val cf = StreamingConnectionFactory(opts)
        val sc = cf.createConnection()

        // Note: Proving an AckHandler can result in NPEs when closing the StreamingConnection
        sc.publish("foo", "Hello World".toByteArray(), AckHandler { nuid, ex ->
            Log.i("Test", "Ack for nuid $nuid and ex $ex")
        })

        uiScope.launch {
            val r = Random()
            var count = 10
            while (count-- > 0) {
                var message = "Value: ${r.nextInt(10000)}"
                // Note: Proving an AckHandler can result in NPEs when closing the StreamingConnection
                sc.publish("foo", "Message $count: ${message}".toByteArray(), AckHandler { nuid, ex ->
                    Log.i("Test", "Ack for nuid $nuid and ex $ex")
                })

                delay(100)
                // Introduce an early close to generate an NPE rather than an IllegalStateException
                sc.close()
            }
            sc.close()
        }