Fix #115775: Curve object to mesh conversion crash

Caused by 4bcdc57.
These objects are "half evaluated" in a very ugly way and have
no runtime data.

source/blender/blenkernel/intern/mesh_convert.cc

@@ -690,8 +690,12 @@ static void curve_to_mesh_eval_ensure(Object &object)
    * Brecht says hold off with that. */
   BKE_displist_make_curveTypes(nullptr, nullptr, &object, true);
 
-  BKE_object_runtime_free_data(&bevel_object);
-  BKE_object_runtime_free_data(&taper_object);
+  if (bevel_object.runtime) {
+    BKE_object_runtime_free_data(&bevel_object);
+  }
+  if (taper_object.runtime) {
+    BKE_object_runtime_free_data(&taper_object);
+  }
 }
 
 static const Curves *get_evaluated_curves_from_object(const Object *object)