[bpo-34658](https://www.bugs.python.org/issue34658): Fix a rare interpreter unhandled exception state SystemError only
seen when using subprocess with a preexec_fn while an after_parent handler has
been registered with os.register_at_fork and the fork system call fails.

https://bugs.python.org/issue34658
(cherry picked from commit a20b6adb)
Co-authored-by: Gregory P. Smith <greg@krypto.org>