"Video speed up" tool generates non-valid media

[Disaster2life]

Please include the following information.

1. Flowblade version (Help->About): 2.12.0.2
2. MLT version (Help->Runtime Environment): 7.20.0
3. Your distribution (Ubuntu, Debian, Mint etc.): Debian Unstable (Unrelated to bug report, shouldn't we update the format since the project only packages for flatpak and the user distro is not essential to resolving issues?)
4. If possible, try to provide step-by-step instructions on how to reproduce the issue. From developer point of view this is the easieast way to understand what is happening.:

• Select Clip
• Go through to the video speed up option in the menu
• Render a sped up file Generates in an error prompt after the media is tried to be brought into the project, returns an error via a prompt video "Can't Open non-valid media" "video file gives value 0 for 'fps_den' property." then it gives a path to the file and says its not a valid media file.

Opening in VLC gives me audio, but no video.

[jliljebl]

This didn't reproduce on my system. I'll leave this open if we get more reports. I'll also do a test with different version of MLT later to get that angle cleared, sometimes MLT versions have different behaviours.