MINOR: mux-h2/trace: report h2s->id before h2c->dsi for the stream ID

h2c->dsi is only for demuxing, and needed while decoding a new request.
But if we already have a valid stream ID (e.g. response or outgoing
request), we should use it instead. This avoids seeing [0] in front of
the responses at user level.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 1023d20..7b4f085 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -532,13 +532,13 @@
 
 		if (type == HTX_BLK_REQ_SL)
 			chunk_appendf(&trace_buf, " : [%d] H2 REQ: %.*s %.*s %.*s",
-				      h2c->dsi,
+				      h2s ? h2s->id : h2c->dsi,
 				      HTX_SL_P1_LEN(sl), HTX_SL_P1_PTR(sl),
 				      HTX_SL_P2_LEN(sl), HTX_SL_P2_PTR(sl),
 				      HTX_SL_P3_LEN(sl), HTX_SL_P3_PTR(sl));
 		else if (type == HTX_BLK_RES_SL)
 			chunk_appendf(&trace_buf, " : [%d] H2 RES: %.*s %.*s %.*s",
-				      h2c->dsi,
+				      h2s ? h2s->id : h2c->dsi,
 				      HTX_SL_P1_LEN(sl), HTX_SL_P1_PTR(sl),
 				      HTX_SL_P2_LEN(sl), HTX_SL_P2_PTR(sl),
 				      HTX_SL_P3_LEN(sl), HTX_SL_P3_PTR(sl));