BUG/MINOR: mux_quic: prevent QMux crash on qcc_io_send() error path

BUG/MINOR: mux_quic: prevent QMux crash on qcc_io_send() error path

A QCC connection may be flagged with QC_CF_ERRL to trigger a
CONNECTION_CLOSE emission. However, for now error reporting is not
functional with QMux, as it relies on quic_conn layer access.

To prevent a crash in qcc_io_send() when using QMux, add a
conn_is_quic() check when QC_CF_ERRL is set to ensure no access will be
performed on quic_conn layer. In the future, this should be extended so
that QMux is also able to emit CONNECTION_CLOSE for connection closure.

No need to backport.