MINOR: mux-h2/trace: missing conn pointer in demux full message

One trace was missing the connection's pointer, reporting "demux buffer full"
without indicating for what connection it was.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 2fca7ac..816cddc 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -3310,7 +3310,7 @@
 
 	if (b_data(buf) == buf->size) {
 		h2c->flags |= H2_CF_DEM_DFULL;
-		TRACE_STATE("demux buffer full", H2_EV_H2C_RECV|H2_EV_H2C_BLK);
+		TRACE_STATE("demux buffer full", H2_EV_H2C_RECV|H2_EV_H2C_BLK, h2c->conn);
 	}
 
 	TRACE_LEAVE(H2_EV_H2C_RECV, h2c->conn);