BUG/MINOR: h3: fix frame demuxing

BUG/MINOR: h3: fix frame demuxing

The H3 demuxing code was not fully correct. After parsing the H3 frame
header, the check between frame length and buffer data is wrong as we
compare a copy of the buffer made before the H3 header removal.

Fix this by improving the H3 demuxing code API. h3_decode_frm_header()
now uses a ncbuf instance, this prevents an unnecessary cast
ncbuf/buffer in h3_decode_qcs() which resolves this error.

This bug was not triggered at this moment. Its impact should be really
limited.