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