The armature reported had two bones with a `bMotionPath` but its lengths
are zero (which causes trouble in `motion_path_cache` drawing code due
to 0 allocations).
Not exactly sure how we got there, something like
`animviz_verify_motionpaths` should take care of this already in current
code, but this might be from a time where there were not enough sanity
checks.
So now early out in `motion_path_cache` if we encounter such a
"corrupted" motion path.
Philipp Oeser