Bug 373480

Steps to reproduce:
- open kdenlive
- add 2 clips
- add them to timeline one after one
- add 'Mute' effect to the second clip
- add 'Speed' effect to the same clip
- set Speed to 50%
- hold Shift, select all 2 clips and move them to right
- set cursor near the end of the second clip

Reuslt: black frame in preview
Expected result: frame of the second clip appeared in preview

Please see the video demonstration of the bug here: https://youtu.be/TmeZxu52EJg

You can also save the project, then close and open it and see that the speed effect disappeared. That's why there was no preview for the clip after its half (speed 50%).

OS: Ubuntu MATE 16.04 64bit
DE: MATE 1.12.1
Locale: be_BY
Hardware: ASUS G550JK-CN287H, Intel videocard
Kdenlive version: 16.08.2
MLT version: 6.3.0+git201610101603~ubuntu16.04.1
Kdenlive package and its dependencies are from this repo: https://launchpad.net/~kdenlive/+archive/ubuntu/kdenlive-stable
Kdenlive project settings: Full HD 1080, framerate 29.72

But this bug also exists in kdenlive 15.12.3 and mlt 6.0.0 (versions from default Ubuntu 16.04 repos) and in kdenlive-devel installed from snap package.