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.