Fix TrampolineScheduler NullPointerException

I tried for about 30 minutes to replicate the NPE reported in ReactiveX#1702 but couldn't.
It makes sense reading the code that an unsubscribe could trigger an NPE though so I'm fixing it as per recommendation of @DylanSale even though I can't replicate.
I confirmed that the items are being put in the queue BEFORE the wip variable is incremented, so that concurrency seems okay.

src/main/java/rx/schedulers/TrampolineScheduler.java

@@ -82,7 +82,11 @@ private Subscription enqueue(Action0 action, long execTime) {
 
             if (wip.getAndIncrement() == 0) {
                 do {
-                    queue.poll().action.call();
+                    TimedAction polled = queue.poll();
+                    // check for null as it could have been unsubscribed and removed
+                    if (polled != null) {
+                        polled.action.call();
+                    }
                 } while (wip.decrementAndGet() > 0);
                 return Subscriptions.empty();
             } else {