The converter can still hold readagain bytes from an earlier rbuf fill,
so fill_cbuf must not put all of them back into the current rbuf. Clamp
the putback to the bytes taken from the current fill; the rest stay in
the converter and are re-read first on the next conversion.
Co-authored-by: Percze Dániel <162882837+Flashmyname@users.noreply.github.com>
Co-Authored-By: Claude Mythos 5.1 <noreply@anthropic.com>