BUG/MINOR: quic: acknowledge STREAM frame even if MUX is released

When the MUX is freed, the quic-conn layer may stay active until all
streams acknowledgment are processed. In this interval, if a new stream
is opened by the client, the quic-conn is thus now responsible to handle
it. This is done by the emission of a STOP_SENDING + RESET_STREAM.

Prior to this patch, the received packet was not acknowledged. This is
undesirable if the quic-conn is able to properly reject the request as
this can lead to unneeded retransmission from the client.

This must be backported up to 2.6.
diff --git a/src/quic_conn.c b/src/quic_conn.c
index 81b1119..9661cc6 100644
--- a/src/quic_conn.c
+++ b/src/quic_conn.c
@@ -2958,11 +2958,16 @@
 				else {
 					TRACE_DEVEL("No mux for new stream", QUIC_EV_CONN_PRSHPKT, qc);
 					if (qc->app_ops == &h3_ops) {
-						if (!qc_h3_request_reject(qc, stream->id))
-							TRACE_ERROR("could not enqueue STOP_SENDING frame", QUIC_EV_CONN_PRSHPKT, qc);
+						if (!qc_h3_request_reject(qc, stream->id)) {
+							TRACE_ERROR("error on request rejection", QUIC_EV_CONN_PRSHPKT, qc);
+							/* This packet will not be acknowledged */
+							goto leave;
+						}
+					}
+					else {
+						/* This packet will not be acknowledged */
+						goto leave;
 					}
-					/* This packet will not be acknowledged */
-					goto leave;
 				}
 			}