Blender Git Loki

Fix regression in recent change 0708733c46cb6b50697c1b80a6541d6fb410267e

Adding a mirror modifier in edit-mode crashed.

Freeing meshes that hold a shallow copy happens in multiple places
while calculating modifiers, making it impractical to clear the
edit-mode pointer before freeing the mesh (as done in
BKE_editmesh_free_derived_caches).

Add a struct member to the edit-mesh struct so evaluated copies
don't free the edit-mesh contents.