fixed #5285 fatalError when connection was lost immediately after sending r…

[jollyjinx]

fixes #5285
I tried to write a test case, but it's a hard to reproduce error as it will not happen when just closing the connection from the server side. But as it happens in the wild and crashes it should behave correctly with this fix.

[lxbndr]

I can confirm this really happens. We had to patch it in our fork almost same way. We're using NSURLErrorUnknown code, but I think your guess is better, and NSURLErrorNetworkConnectionLost fits well for this state. And yes, we also have no idea how could this be triggered manually. So we have no tests, and did no attempt merging the patch to upstream. Would be really nice if this become accepted and landed.

Just nitpicking here, @jollyjinx, would you mind adjusting indentation? Alignment by first argument, like in similar error below your lines.

[compnerd]

@swift-ci please test

[parkera]

@guoye-zhang please review

[guoye-zhang]

The change looks fine, but would be good to figure out how to reproduce this since there might be an underlying bug of the response getting lost