2.x: fix Flowable.toList() onNext/cancel race

[akarnokd]

This PR fixes the NullPointerException that can happen because cancel nulls out the value and onNext simply dereferences that value in FlowableToList.

Reported in #5246 .

[codecov]

Codecov Report

Merging #5247 into 2.x will decrease coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x   #5247      +/-   ##
===========================================
- Coverage     96.04%     96%   -0.05%     
- Complexity     5749    5750       +1     
===========================================
  Files           628     628              
  Lines         41073   41075       +2     
  Branches       5697    5698       +1     
===========================================
- Hits          39450   39434      -16     
- Misses          636     659      +23     
+ Partials        987     982       -5

Impacted Files	Coverage Δ	Complexity Δ
...ex/internal/operators/flowable/FlowableToList.java	96.77% <100%> (+0.22%)	2 <0> (ø)	⬇️
...rnal/subscriptions/ArrayCompositeSubscription.java	93.33% <0%> (-6.67%)	17% <0%> (-1%)
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-6.56%)	2% <0%> (ø)
.../java/io/reactivex/subjects/SerializedSubject.java	89.47% <0%> (-6.32%)	26% <0%> (-1%)
...in/java/io/reactivex/subjects/BehaviorSubject.java	84.97% <0%> (-5.7%)	56% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	88.54% <0%> (-3.97%)	62% <0%> (+1%)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
... and 42 more

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