Fix: Potential crash in mesh comparison tests with mismatched attributes

Currently we allow the before/after meshes to have different
sets of attributes. But then we compare all attributes in each
mesh anyway, regardless of whether they exist or not.

source/blender/blenkernel/intern/mesh_compare.cc

@@ -522,6 +522,10 @@ static std::optional<MeshMismatch> verify_attributes_compatible(
   for (const StringRef id : mesh1_attribute_ids) {
     GAttributeReader reader1 = mesh1_attributes.lookup(id);
     GAttributeReader reader2 = mesh2_attributes.lookup(id);
+    if (!reader1 || !reader2) {
+      /* Necessary because of previous disabled return. */
+      continue;
+    }
     if (reader1.domain != reader2.domain || reader1.varray.type() != reader2.varray.type()) {
       return MeshMismatch::AttributeTypes;
     }