Two issues:
* Automatic deduplication of OpenVDB grid data was failing when Cycles had
already cleared the OpenVDB grid, causing an empty grid. Instead rely on
Blender return the same OpenVDB grid pointer when deduplication is possible.
* The volume bounds mesh was not properly cleared when the OpenVDB grid was
empty, causing a mismatch between mesh and voxel data.
5c01ecd2bf