[php-src] PR #23594: iconv: keep working converter when stream filter seek reset fails

Pull Request: https://github.com/php/php-src/pull/23594 Author: iliaal php_iconv_stream_filter_seek() closed the filter's converter before knowing whether a replacement could be opened, then assigned the failed iconv_open() result. That left self->cd as (iconv_t)-1 for every subsequent read and write, and the filter dtor then called iconv_close() on it. Opening the replacement first and swapping it in only on success keeps the working converter on failure. No test: the charsets are fixed when the filter is created and already validated by the iconv_open() there, so a second open of the same pair can only fail on OOM. There is no userland input that reaches the branch.