[3.6] bpo-31804: Fix multiprocessing.Process with broken standard streams (GH-6079...

[3.6] bpo-31804: Fix multiprocessing.Process with broken standard streams (GH-6079) (GH-6081)

In some conditions the standard streams will be None or closed in the child process (for example if using "pythonw" instead of "python" on Windows).  Avoid failing with a non-0 exit code in those conditions.

Report and initial patch by poxthegreat..
(cherry picked from commit e756f66c83786ee82f5f7d45931ae50a6931dd7f)