There seems to not be a strict rule to only have armatures as parents to
single bones, apparently collada files can also be set up to have mesh
parents for bones. As a consequence, the collada importer
`joint_parent_map` is not safe to fetch objects from and assume their
data can be cast to `bArmature`.
Ultimately, the `joint_parent_map` needs to be looked at again, this
patch just avoids a crash in this scenario (so no such joints will be
iported). Still better than crashing I guess.
Pull Request: https://projects.blender.org/blender/blender/pulls/118751
f880cbce93