fix to terminate the process that has ended

[luyanger1799]

check if the process exists before using teminate() to stop the process.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it
• I have formatted my code using black -t py36

[coveralls]

Coverage decreased (-0.01%) to 67.532% when pulling 1c555ab on luyanger1799:master into 79b2c1d on Zulko:master.

[tburrows13]

Hello, thanks for the PR. I'm not quite sure why this is necessary? Does calling terminate crash if the process has already terminated?

[mondeja]

I'm seeing this legit 👍 Thank you for the improvement @luyanger1799 🙏

If the process has been stopped by another program or the user want to interact directly with the project, this becomes useful.

Does calling terminate crash if the process has already terminated?

Nope

[tburrows13]

I don't really understand, so I'll go with your judgement on this. Presumably the same thing should be added to https://github.com/Zulko/moviepy/blob/master/moviepy/video/io/ffmpeg_reader.py#L228 then?

You can commit directly to this PR if you need to, so that it is done as one PR.