Race condition in reduce with seed cancellation can lead to NPE

[simonbasle]

Expected behavior

A reducing Flux with a seed value can safely be cancelled.

Actual behavior

The BiFunction can be fed a null value in case there is a concurrent cancel() along the first onNext(T). This is due to cancel nulling out the internal accumulated value and onNext not checking for nulls.

Steps to reproduce

	@Test
	public void onNextAndCancelRace() {
		List<Integer> list = new ArrayList<>();
		final AssertSubscriber<Object> testSubscriber = AssertSubscriber.create();

		MonoReduceSeed.ReduceSeedSubscriber<Integer, List<Integer>> sub =
				new MonoReduceSeed.ReduceSeedSubscriber<>(testSubscriber,
						(l, next) -> {
							l.add(next);
							return l;
						}, list);

		sub.onSubscribe(Operators.emptySubscription());
		sub.cancel();
		sub.onNext(1);
		testSubscriber.assertNoError(); //=> fails, NullPointerException in lambda above
	}

Reactor Core version

3.1.10.RELEASE