Fix regression in recent change 0708733c46

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.

source/blender/blenkernel/BKE_editmesh.h

@@ -78,6 +78,11 @@ typedef struct BMEditMesh {
   /** Temp variables for x-mirror editing (-1 when the layer does not exist). */
   int mirror_cdlayer;
 
+  /**
+   * Enable for evaluated copies, causes the edit-mesh to free the memory, not it's contents.
+   */
+  char is_shallow_copy;
+
   /**
    * ID data is older than edit-mode data.
    * Set #Main.is_memfile_undo_flush_needed when enabling.

source/blender/blenkernel/intern/editmesh.c

@@ -211,14 +211,10 @@ void BKE_editmesh_looptri_and_normals_calc_with_partial(BMEditMesh *em,
 void BKE_editmesh_free_derived_caches(BMEditMesh *em)
 {
   if (em->mesh_eval_cage) {
-    Mesh *me = em->mesh_eval_cage;
-    me->edit_mesh = NULL;
-    BKE_id_free(NULL, me);
+    BKE_id_free(NULL, em->mesh_eval_cage);
   }
   if (em->mesh_eval_final && em->mesh_eval_final != em->mesh_eval_cage) {
-    Mesh *me = em->mesh_eval_final;
-    me->edit_mesh = NULL;
-    BKE_id_free(NULL, me);
+    BKE_id_free(NULL, em->mesh_eval_final);
   }
   em->mesh_eval_cage = em->mesh_eval_final = NULL;
 

source/blender/blenkernel/intern/mesh.c

@@ -161,7 +161,9 @@ static void mesh_free_data(ID *id)
   BLI_freelistN(&mesh->vertex_group_names);
 
   if (mesh->edit_mesh) {
-    BKE_editmesh_free_data(mesh->edit_mesh);
+    if (mesh->edit_mesh->is_shallow_copy == false) {
+      BKE_editmesh_free_data(mesh->edit_mesh);
+    }
     MEM_freeN(mesh->edit_mesh);
     mesh->edit_mesh = NULL;
   }

source/blender/blenkernel/intern/mesh_wrapper.c

@@ -69,7 +69,9 @@ Mesh *BKE_mesh_wrapper_from_editmesh_with_coords(BMEditMesh *em,
 
   /* Use edit-mesh directly where possible. */
   me->runtime.is_original = true;
-  me->edit_mesh = em;
+
+  me->edit_mesh = MEM_dupallocN(em);
+  me->edit_mesh->is_shallow_copy = true;
 
 /* Make sure, we crash if these are ever used. */
 #ifdef DEBUG