Blender Git Loki

Fix T90387: division by zero when trying to invert scale

Fix division by zero when `BKE_bone_parent_transform_invert()` inverts a
scale vector with zero components.

Zero values in the to-be-inverted vector are now simply skipped, i.e.
remain zero after inversion. This at least ensures that
`invert_v3_safe(invert_v3_safe(vector))` results in the same vector.

This commit does NOT fix the conceptual problem that an inversion of a
potentially non-invertible vector is relied upon. It just avoids the
division by zero.