CancellationToken in ToAsyncEnumerable can cause NullReferenceException

#### Bug

It look like there is a race condition in `ToAsyncEnumerable` which can cause `NullReferenceException` when the cancellation token is requested.

> Which subcomponent library (Ix, Async.Ix)?

Ix

> Which library version?

System.Linq.Async 6.0.1

> What are the platform(s), environment(s) and related component version(s)?

Windows, .NET6

> What is the use case or problem?

Cancellation of `ObservableAsyncEnumerable`.

> What is the expected outcome?

`OperationCanceledException` is thrown.

> What is the actual outcome?

`NullReferenceException` is thrown intermittently.

> What is the stacktrace of the exception(s) if any?

```
---> System.NullReferenceException: Object reference not set to an instance of an object.
at System.Linq.AsyncEnumerable.ObservableAsyncEnumerable`1.MoveNextCore() in /_/Ix.NET/Source/System.Linq.Async/System/Linq/Operators/ToAsyncEnumerable.Observable.cs:line 88
at System.Linq.AsyncIteratorBase`1.MoveNextAsync() in /_/Ix.NET/Source/System.Linq.Async/System/Linq/AsyncIterator.cs:line 70
at System.Linq.AsyncIteratorBase`1.MoveNextAsync() in /_/Ix.NET/Source/System.Linq.Async/System/Linq/AsyncIterator.cs:line 75
at Program.<$>g__ConsumeAsync|0_2(Subject`1 subject, CancellationToken ct) in C:\Users\shara\source\repos\ToAsyncEnumerableTest\Program.cs:line 30
```

> Do you have a code snippet or project that reproduces the problem?

```
using System.Reactive.Subjects;

Parallel.For(1, 10, _ => RunAsync().Wait());

async Task RunAsync()
{
// Set custom context to avoid sychronous execution of enumerable continuation
SynchronizationContext.SetSynchronizationContext(new CustomSynchronizationContext());

using var subject = new Subject();
using var cts = new CancellationTokenSource();

var task = ConsumeAsync(subject, cts.Token);

subject.OnNext(1);
cts.Cancel();

try
{
await task;
}
catch (OperationCanceledException)
{
// ignore
}
}

async Task ConsumeAsync(Subject subject, CancellationToken ct)
{
await foreach (var _ in subject.ToAsyncEnumerable().WithCancellation(ct))
{
}
}

internal class CustomSynchronizationContext : SynchronizationContext
{
}
```