MINOR: mux-h2: always pass HTX_FL_PARSING_ERROR between h2s and buf on RX

In order to allow the H2 parser to report parsing errors, we must make
sure to always pass the HTX_FL_PARSING_ERROR flag from the h2s htx to
the conn_stream's htx.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index b641d22..fe4d092 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -5138,6 +5138,10 @@
 		}
 
 		htx_ret = htx_xfer_blks(buf_htx, h2s_htx, count, HTX_BLK_EOM);
+
+		if (h2s_htx->flags & HTX_FL_PARSING_ERROR)
+			buf_htx->flags |= HTX_FL_PARSING_ERROR;
+
 		buf_htx->extra = (h2s_htx->extra ? (h2s_htx->data + h2s_htx->extra) : 0);
 		htx_to_buf(buf_htx, buf);
 		htx_to_buf(h2s_htx, &h2s->rxbuf);