Previously, hair strands of zero length of too few control
points would have been ignored. This is fine for a render
without motion blur. But once motion blur is enabled it is
becoming more tricky to match topology.
Even more, it was causing access (and possibly writes) past
the array boundaries in case when time step 0 ignored some
strands and steps around it did not.
If this is becoming problematic for BVH to do reliable
intersections this is to be solved on the BVH builder side.
The export from Blender to Cycles shouldn't really make
decisions there.
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