Avoid "IOException: cannot find current thread" when submitting an input step that ends a CpsThread

[dwnusbaum]

Fixes a timing issue in #145 - whenever the input step was the last part of a CpsThread, it was possible that the thread would be removed from the thread group before we called StepContext.get, causing CpsThreadGroup.doGet to throw an exception.

jenkinsci/workflow-support-plugin#252 would make this kind of change obsolete, but this change is minimal and cannot affect other plugins, so I think worth considering as a hotfix.

Testing done

I tested this manually using a test that failed reliably after jenkinsci/workflow-support-plugin#252. With this change, it passes reliably.

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[jglick]

Looks safe.