MINOR: mux_quic: handle conn errors on QMux without crash

MINOR: mux_quic: handle conn errors on QMux without crash

Remove BUG_ON() related to connection errors when invoking XPRT
snd_buf/rcv_buf in QMux operations. Such errors are now converted in
QC_CF_ERR_CONN flag, which will disable any I/O operations and close the
connection as soon as possible.

Note that this error management is pretty crude. In particular, it could
lead to truncated data when dealing with unidirectional connection
closure from the remote peer. However, it is considered sufficient for
now to continue interop testing without being disturbed by BUG_ON()
assertion crashes.