BUG/MINOR: quic: Null packet dereferencing from qc_dup_pkt_frms() trace

This function must duplicate frames be resent from packets. Some of
them are still in flight, others have already been detected as lost.
In this case the original frame ->pkt member is NULL.
Add a trace to distinguish these cases.

Thank you to Tristan for having reported this issue in GH #1808.

Must be backported to 2.6.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 0aad272..f5dedca 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -2355,8 +2355,13 @@
 		 */
 		origin = frm->origin ? frm->origin : frm;
 		TRACE_DEVEL("built probing frame", QUIC_EV_CONN_PRSAFRM, qc, origin);
-		TRACE_DEVEL("duplicated from packet", QUIC_EV_CONN_PRSAFRM,
-		            qc, NULL, &origin->pkt->pn_node.key);
+		if (origin->pkt)
+			TRACE_DEVEL("duplicated from packet", QUIC_EV_CONN_PRSAFRM,
+			            qc, NULL, &origin->pkt->pn_node.key);
+		else {
+			/* <origin> is a frame which was sent from a packet detected as lost. */
+			TRACE_DEVEL("duplicated from lost packet", QUIC_EV_CONN_PRSAFRM, qc);
+		}
 		*dup_frm = *origin;
 		dup_frm->pkt = NULL;
 		dup_frm->origin = origin;