BUG/MINOR: quic: fix closing state on NO_ERROR code sent

Reception is disabled as soon as a CONNECTION_CLOSE emission is
required. An early return is done on qc_lstnr_pkt_rcv() to implement
this.

This condition is not functional if the error code sent is NO_ERROR
(0x00). To fix this, check the quic-conn flags instead of the
error code.

Currently this bug has no impact has NO_ERROR emission is not used.

This can be backported up to 2.6.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 34fc13e..88cf4fd 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -5571,7 +5571,7 @@
 
 	dgram->qc = qc;
 
-	if (qc->err_code) {
+	if (qc->flags & QUIC_FL_CONN_IMMEDIATE_CLOSE) {
 		TRACE_PROTO("Connection error",
 		            QUIC_EV_CONN_LPKT, qc, NULL, NULL, qv);
 		goto out;