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.
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 8c9c051..bc8910c 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -1222,7 +1222,7 @@
 
 static void qc_detach(struct cs_endpoint *endp)
 {
-	struct qcs *qcs = __cs_mux(endp->cs);
+	struct qcs *qcs = endp->target;
 	struct qcc *qcc = qcs->qcc;
 
 	TRACE_ENTER(QMUX_EV_STRM_END, qcc->conn, qcs);