Fix #106901: Dangling pointer after freeing mesh runtime data

This lead to a double-free when deleting the mesh runtime
struct after explicitly clearing the derived caches separately.

source/blender/blenkernel/intern/mesh_runtime.cc

@@ -224,6 +224,7 @@ void BKE_mesh_runtime_clear_geometry(Mesh *mesh)
   mesh->runtime->subsurf_optimal_display_edges.clear_and_shrink();
   if (mesh->runtime->shrinkwrap_data) {
     BKE_shrinkwrap_boundary_data_free(mesh->runtime->shrinkwrap_data);
+    mesh->runtime->shrinkwrap_data = nullptr;
   }
 }