Blender Git Loki

Fix T84142: crash when mirroring hair emitted from vertices

The hair mirroring code seems to expect that hair is emitted from faces.
The PE_mirror_x contains the following expression: mirrorfaces[pa->num * 2].
This only makes sense when pa->num is a face index.

The simplest short term solution is to disable the mirror operator when
the particles haven't been emitted from faces.

Diffferential Revision: https://developer.blender.org/D10002