Ignore dead threads in coroutine_join.

[ioquatix]

@jeremyevans

I'm preparing a PR and noticed some failing specs in the pthread fiber implementation: https://github.com/jeremyevans/ruby/runs/2965165330#step:15:128. Looks like it also happened before my PR: https://github.com/ruby/ruby/runs/2961125088?check_suite_focus=true#step:15:131. These aren't failing every time, so the failure must be non-deterministic.

[jeremyevans]

Not my area of expertise, but the change does make sense to me.

[ioquatix]

This fix may be insufficient, but let's try it.

It may be necessary to explicitly walk all fibers and put them into an invalid state after forking, in the child process. This deviates from the native fiber implementation where fibers will continue to work after fork AFAIK.

[ioquatix]

@nobu thanks for mentioning the CI failure, this is a first step to address it.