BUG/MAJOR: mux_quic: fix invalid PROTOCOL_VIOLATION on POST data overlap

BUG/MAJOR: mux_quic: fix invalid PROTOCOL_VIOLATION on POST data overlap

Stream data reception is incorrect when dealing with a partially new
offset with some data already consumed out of the RX buffer. In this
case, data length is adjusted but not the data buffer. In most cases,
ncb_add() operation will be rejected as already stored data does not
correspond with the new inserted offset. This will result in an invalid
CONNECTION_CLOSE with PROTOCOL_VIOLATION.

To fix this, buffer pointer is advanced while the length is reduced.

This can be reproduced with a POST request and patching haproxy to call
qcc_recv() multiple times by copying a quic_stream frame with different
offsets.

Must be backported to 2.6.