Fix TrampolineScheduler NullPointerException

I tried for about 30 minutes to replicate the NPE reported in https://github.com/ReactiveX/RxJava/issues/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.
Remove use of ThreadLocal

This was an awkward leftover from early version of RxJava (pre v0.17).
Merge pull request #1736 from benjchristensen/issue-1702

Fix TrampolineScheduler NullPointerException