BUILD: quic: fix potential NULL dereference on xprt_quic

A warning is triggered by gcc9 on this code path, which is the compiler
version used by ubuntu20.04 on the github CI.

This is linked to github issue #1445.
diff --git a/src/quic_sock.c b/src/quic_sock.c
index a574de0..38a8aab 100644
--- a/src/quic_sock.c
+++ b/src/quic_sock.c
@@ -191,9 +191,9 @@
 		return;
 
 	rxbuf = MT_LIST_POP(&l->rx.rxbuf_list, typeof(rxbuf), mt_list);
-	buf = &rxbuf->buf;
-	if (!buf)
+	if (!rxbuf)
 		goto out;
+	buf = &rxbuf->buf;
 
 	params = &l->bind_conf->quic_params;
 	max_sz = params->max_udp_payload_size;