MINOR: mux-quic: check quic-conn return code on Tx

Inspect return code of qc_send_mux(). If quic-conn layer reports an
error, this will interrupt the current emission process.

This should be backported up to 2.6.
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 0482e92..3baab76 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -1399,7 +1399,8 @@
 
 	LIST_INIT(&qcc->send_retry_list);
 
-	qc_send_mux(qcc->conn->handle.qc, frms);
+	if (!qc_send_mux(qcc->conn->handle.qc, frms))
+		goto err;
 
 	/* If there is frames left at this stage, transport layer is blocked.
 	 * Subscribe on it to retry later.