BUG/MINOR: mux-quic: do not decode if conn in error

BUG/MINOR: mux-quic: do not decode if conn in error

Add an early return to qcc_decode_qcs() if QCC instance is flagged on
error and connection is scheduled for immediate closure.

The main objective is to ensure to not trigger BUG_ON() from
qcc_set_error() : if a stream decoding has set the connection error, do
not try to process decoding on other streams as they may also encounter
an error. Thus, the connection is closed asap with the first encountered
error case.

This should be backported up to 2.6, after a period of observation.