The example file in T81218 has a driver that maps a bone's X-location to
the number of BBone segments. This caused a dependency cycle, which
resulted in bad thread serialisation, which caused the crash.
This patch breaks the dependency cycle `BONE_LOCAL` →
`DRIVER(bones["Bone"].bbone_segments)` → `BONE_LOCAL`. The 'Driver Data'
relation now points to `BONE_SEGMENTS` when the driven property starts
with `bbone_`.
Differential Revision: https://developer.blender.org/D9122
d43e3f34d2