Fix NPE when iterable is null

As per #1676, I prefer that throwing an NullPointerException in the constructor of `OnSubscribeFromIterable`. 

BTW, because https://github.com/ReactiveX/RxJava/blob/v1.0.0-rc.5/src/main/java/rx/internal/operators/OnSubscribeFromIterable.java#L43 does not return after `o.onCompleted();`, there is still a NPE bug here.