3.x: Fix concurrent clear in observeOn while output-fused

[akarnokd]

There was another cancel-clear race leading to NPE or infinite loop inside both observeOn implementations.

Related: #6676

[codecov]

Codecov Report

Merging #6708 into 3.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               3.x    #6708      +/-   ##
===========================================
- Coverage     98.1%   98.08%   -0.03%     
+ Complexity    6188     6185       -3     
===========================================
  Files          677      677              
  Lines        44657    44657              
  Branches      6170     6170              
===========================================
- Hits         43810    43800      -10     
- Misses         309      313       +4     
- Partials       538      544       +6

Impacted Files	Coverage Δ	Complexity Δ
...internal/operators/flowable/FlowableObserveOn.java	96.65% <100%> (ø)	3 <0> (ø)	⬇️
...rnal/operators/observable/ObservableObserveOn.java	100% <100%> (ø)	3 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.44% <0%> (-2.91%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-2.11%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-1.61%)	5% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	91.26% <0%> (-1.59%)	2% <0%> (ø)
...ivex/rxjava3/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
... and 14 more

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