Blender Git Loki

Fix T73304: Crash using force fields and hair dynamics

This implements a better heuristic for identifying if cloth or hair is
being dealt with (checking hairdata, instead of primitive_num).

The issue was caused by a change in primitive counting in rBd42a7bbd6ea5

I'm also adding some safeguards to avoid ever computing pressure for
hair. This shouldn't really be necessary, but it's good to be sure.