MINOR: quic: Unitialized mux context upon Client Hello message receipt.

If we let the connection packet handler task (quic_conn_io_cb) process the first
client Initial packet which contain the TLS Client Hello message before the mux
context is initialized, quic_mux_transport_params_update() makes haproxy crash.
->start xprt callback already wakes up this task and is called after all the
connection contexts are initialized. So, this patch do not wakes up quic_conn_io_cb()
if the mux context is not initialized (this was already the case for the connection
context (conn_ctx)).
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 5bc3495..eec3b5c 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -3441,8 +3441,13 @@
 
 
 	TRACE_PROTO("New packet", QUIC_EV_CONN_LPKT, qc->conn, pkt);
-	if (conn_ctx)
-		/* Wake the tasklet of the QUIC connection packet handler. */
+	/* Wake up the connection packet handler task from here only if all
+	 * the contexts have been initialized, especially the mux context
+	 * conn_ctx->conn->ctx. Note that this is ->start xprt callback which
+	 * will start it if these contexts for the connection are not already
+	 * initialized.
+	 */
+	if (conn_ctx && HA_ATOMIC_LOAD(&conn_ctx->conn->ctx))
 		tasklet_wakeup(conn_ctx->wait_event.tasklet);
 
 	TRACE_LEAVE(QUIC_EV_CONN_LPKT, qc->conn, pkt);