Blender Git Loki

Fix Edit Face Set operator not being cancelled when the cursor is not over the mesh

If the cursor is not over the mesh, the operator was still using the
last Face Set ID updated by the drawing cursor code when the cursor was
over the mesh.

This now cancels the operator instead of modifying a Face Set that will
look random to the user.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9545