Fix T39166: Disappearing text objects

This is a failure of viewport direct displist creation
caused by existing curve_cache pointer which empty content.

Made it so if the curve isn't evaluated it's curve_cache is NULL.

This is just-another-regression to be ported to the release.

source/blender/blenkernel/intern/object.c

@@ -331,6 +331,10 @@ void BKE_object_free_derived_caches(Object *ob)
 			free_path(ob->curve_cache->path);
 			ob->curve_cache->path = NULL;
 		}
+
+		/* Signal for viewport to run DAG workarounds. */
+		MEM_freeN(ob->curve_cache);
+		ob->curve_cache = NULL;
 	}
 }