fa0f295b53 exposed that there was a region missing because of some
faulty versioning.
The `fluid_motion_blur.blend` Cycles test file was crashing. Seems like
this is a pre 2.5 file that was resaved in newer versions, but the
preview region for the Sequencer was missing. Before mentioned commit,
the region would be broken when activating the preview view mode in the
Sequencer, now it would crash even on startup.
Ensure the region exists, as expected.
56640440f0