Core: Fix broken CustomData IO with Autosave files

Autosave files are created from memfile undo, which
doesn't save legacy mesh data. This leads to a crash
on file load.

In addition the mesh code can now add CustomData
layers when saving files, which did not work if
the original domain had no layers.  In that case
CustomData.layers is NULL and DNA has nothing to
key off of when loading the file later.

@@ -4220,0 +4220,4 @@
+
+  /* Note: data->layers may be null, this happens when adding
+   * a legacy MPoly struct to a mesh with no other face attributes.
+   * This leaves us with no unique ID for DNA to key off of when

@@ -1243,6 +1243,7 @@ void BKE_mesh_legacy_sharp_faces_from_flags(Mesh *mesh)
   if (attributes.contains("sharp_face")) {
     return;
   }
+

@@ -29,1 +36,4 @@
 {
+  /* Autosave files don't have CD_MPOLY layers.
+   */
+  if (!CustomData_has_layer(&mesh.pdata, CD_MPOLY) && mesh.poly_offset_indices) {

@@ -29,1 +36,4 @@
 {
+  /* Autosave files don't have CD_MPOLY layers.
+   */
+  if (!CustomData_has_layer(&mesh.pdata, CD_MPOLY) && mesh.poly_offset_indices) {

@@ -4220,0 +4225,4 @@
+   */
+  if (!data.layers && layers_to_write.size() > 0) {
+    /* We just need an address that's unique. */
+    data.layers = reinterpret_cast<CustomDataLayer *>(&data.layers);

@@ -8,21 +8,28 @@
 
 #include "CLG_log.h"
 
+#include "DNA_mesh_types.h"

@@ -4220,0 +4225,4 @@
+   */
+  if (!data.layers && layers_to_write.size() > 0) {
+    /* We just need an address that's unique. */
+    data.layers = reinterpret_cast<CustomDataLayer *>(&data.layers);

@@ -4220,0 +4225,4 @@
+   */
+  if (!data.layers && layers_to_write.size() > 0) {
+    /* We just need an address that's unique. */
+    data.layers = reinterpret_cast<CustomDataLayer *>(&data.layers);