Fix exception when trying to send SIGSTOP to finished sudo child

[laloch]

Fixes #2883.
Although the Python's subprocess module does its best to not send signals to child processes that have already died, the protection is not atomic/race-free. We need to protect ourselves.

[laloch]

Is this worth a news entry?

[scopatz]

Thanks for putting this in @laloch! Yes, please put in a new entry

[melund]

Thanks @laloch. The python 35 test failure is unrelated to this.