MINOR: quic: Drop the packets of discarded packet number spaces

This is required since this previous commit:
   "MINOR: quic: Post handshake I/O callback switching"
If not, such packets remain endlessly in the RX buffer and cannot be parsed
by the new I/O callback used after the handshake has been confirmed.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 9e31a75..314ed2a 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -3144,8 +3144,10 @@
 	tel = ssl_to_quic_enc_level(qel->level);
 
 	/* check if tls secrets are available */
-	if (qel->tls_ctx.rx.flags & QUIC_FL_TLS_SECRETS_DCD)
+	if (qel->tls_ctx.rx.flags & QUIC_FL_TLS_SECRETS_DCD) {
 		TRACE_DEVEL("Discarded keys", QUIC_EV_CONN_TRMHP, qc);
+		return 0;
+	}
 
 	if (!(qel->tls_ctx.rx.flags & QUIC_FL_TLS_SECRETS_SET))
 		return 0;