"Reading/writing memory blocks that are not multiple of tuple size is not allowed on a tuple_ringbuffer!" exception is thrown continuously while setting playback speed of video player to less than 1.

Steps to repro:
1. Open Test scene from the attached project.
2. Select Sphere game object from the Hierarchy.
3. Set playback speed of Video player to less than 1.
4. Play the scene.

Observe the exceptions in console window.

Reproducible on:
2018.2.0a8, 2018.2.0a7, 2018.2.0a6.

Working fine on:
2018.2.0a5, 2018.2.0a1, 2018.1.0b13.

Environment:
Occurring on Windows 10, Windows 7, Mac 10.13, Mac 10.12.