2.x: Fix toFlowable(ERROR) not cancelling upon MBE

[akarnokd]

There was a missing upstream.cancel() for when the overflow happens.

Fixes #7081

[codecov]

Codecov Report

Merging #7084 into 2.x will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@ Coverage Diff @@ ## 2.x #7084 +/- ## ============================================ + Coverage 98.20% 98.26% +0.05%  - Complexity 6346 6352 +6  ============================================ Files 677 677 Lines 45548 45549 +1 Branches 6341 6341 ============================================ + Hits 44731 44757 +26  + Misses 264 248 -16  + Partials 553 544 -9 

Impacted Files	Coverage Δ	Complexity Δ
...perators/flowable/FlowableOnBackpressureError.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.80% <0.00%> (-5.98%)	2.00% <0.00%> (ø%)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0.00%> (-1.67%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0.00%> (-1.64%)	2.00% <0.00%> (ø%)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0.00%> (-0.94%)	2.00% <0.00%> (ø%)
...ex/internal/operators/flowable/FlowableReplay.java	93.92% <0.00%> (-0.84%)	20.00% <0.00%> (ø%)
...perators/single/SingleFlatMapIterableFlowable.java	97.50% <0.00%> (-0.84%)	2.00% <0.00%> (ø%)
...main/java/io/reactivex/subjects/ReplaySubject.java	96.88% <0.00%> (-0.63%)	50.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0.00%> (ø)	5.00% <0.00%> (ø%)
...ternal/operators/flowable/FlowableWindowTimed.java	92.32% <0.00%> (+0.23%)	4.00% <0.00%> (ø%)
... and 14 more

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[vanniktech]

Does Observable behave correctly?

[akarnokd]

This is a backpressure-related issue, so Observable is not affected.