BUG/MINOR: mux-quic: fix crash with traces in qc_detach()

BUG/MINOR: mux-quic: fix crash with traces in qc_detach()

qc_detach() is used to free a qcs as notified by sedesc. If there is no
more stream active and the connection is considered as dead, it will
then be freed. This prevent to dereference qcc in TRACE macro. Else this
will cause a crash.

Use a different code-path on release for qc_detach() to fix this bug.

This will fix the last occurence of crash on github issue #1808.

This has been introduced by recent QUIC MUX traces rework. Thus, it does
not need to be backport.