movie preview bug fix, in some case stream reading functions may block indefinitely

[jpsn123]

Summary

For some reason, I need to run nextcloud as root, and in this case ffmpeg will block the process when it generates preview.

refencese:

https://www.php.net/manual/en/function.proc-open.php#97012
https://www.php.net/manual/en/function.stream-get-contents.php#106905

[blizzz]

From the documentation:

In non-blocking mode an fgets() call will always return right away while in blocking mode it will wait for data to become available on the stream.

Could be more exhaustive. Is it possible that we only receive incomplete data with this setting?

[blizzz]

@jpsn123 thanks for the adaptions! Would you mind rebasing one more time?

[artonge]

@jpsn123 sorry for the delay here. Can you sign off your commit?

git rebase HEAD~1 --signoff
git push --force-with-lease origin master

Then we should be good :)

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22