Missing error check in ast_opt.c

This line was copied from `fold_iter`. So, `fold_iter` has the same issue (no error handling for NULL values on calls to `PyFrozenSet_New`). Do you think it's OK to address this issue in this PR?

_Originally posted by @Eclips4 in https://github.com/python/cpython/pull/123081#discussion_r1722687840_
            


### Linked PRs
* gh-123172