The issue was that the same point cache was read by multiple
threads at the same time (the same object was evaluated for
render and for the viewport).
Both threads incremented PTCacheMem->cur which lead to the crash.
The fix is to remove the PTCacheMem->cur and store it on the
stack instead. This way every thread has its own cur.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D9606
dca36a8ec9