MEDIUM: quic: do not ACK packet with STREAM if MUX not present

MEDIUM: quic: do not ACK packet with STREAM if MUX not present

If a packet contains a STREAM frame but the MUX is not allocated, the
frame cannot be enqueued. According to the RFC9000, we must not
acknowledge the packet under this condition.

This may prevents a bug with firefox which keeps trying on refreshing
the web page. This issue has already been detected before closing state
implementation : haproxy wasn't emitted CONNECTION_CLOSE and keeps
acknowledge STREAM frames despite not handle them.

In the future, it might be necessary to respond with a CONNECTION_CLOSE
if the MUX has already been freed.