Blender Git Loki

Fix(unreported): Crash on prefetching while transforming strip.

While investigating T76274, I found crash scenario when playhead is near end
frame and moving a strip. It is not as easy to reproduce, about 5% success
rate, and it will be even harder after rB4066c921fbe5. Exact cause wasn't
identified yet.

I wanted to disable prefetching during modal operator execution in VSE, but
currently I don't have any signalling method in place. Checking for G.moving
seems to resolve this problem, but it doesn't adress root cause of bug.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D7820