Fix small UI issues

- in EVP_read_pw_string_min(), the return value from UI_add_* wasn't
  properly checked
- in UI_process(), |state| was never made NULL, which means an error
  when closing the session wouldn't be accurately reported.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/3849)