Fix potential crashes in asFlow extensions for RxJava 2&3 integrations

[LouisCAD]

Fixes #2299

Related to issue #974

[mhernand40]

Thanks for taking this. May I request that the fix also be applied to Rx2 as well please? 🙂

[LouisCAD]

Don't you want to do it now that you've seen my example, @mhernand40?

[mhernand40]

@LouisCAD Typically I've seen bug fixes for Coroutines <-> Reactive APIs applied all throughout where applicable in the same PR. Hence why I am asking it be addressed within this PR. But I am also okay with tackling it in a separate PR provided this one gets approved first. Anyhow, FWIW I personally would be grateful if it would be addressed in this PR. 🙂

[LouisCAD]

Done.

[mhernand40]

I'm really hoping this makes it into the final Coroutines 1.4 release. We had a major incident recently due to this issue where app crashes surged in production and we ultimately had to push out a hot fix. I was planning in the meantime to provide our own internal implementation of ObservableSource.asFlow() which would pretty much be a copy/paste of this implementation but I am hoping that updating to 1.4 will be all we need. 🙂

[qwwdfsad]

I've opened #2333 where I squashed all the changes into a single commit and added stable and small reproducer.

Thanks for taking care of this issue!