BUG/MEDIUM: mux-quic: fix a thinko in the latest cs/endpoint cleanup

Fred & Amaury found that I messed up with qc_detach() in commit 4201ab791
("CLEANUP: muxes: make mux->attach/detach take a conn_stream endpoint"),
causing a segv in this case with endp->cs == NULL being passed to
__cs_mux(). It obviously ought to have been endp->target like in other
muxes.

No backport needed.
1 file changed