proxy: ws: handle case onward conn closes during upgrade

Co-developed-by: Gemini 2.5 Pro

Author: lws-team

lib/core-net/dummy-callback.c

@@ -121,6 +121,13 @@ lws_callback_ws_proxy(struct lws *wsi, enum lws_callback_reasons reason,
 		if (!wsi->h1_ws_proxied || !wsi->parent)
 			break;
 
+		/*
+		 * If the parent has started to close, don't try to
+		 * upgrade it, just let it go.
+		 */
+		if (lwsi_state(wsi->parent) >= LRS_RETURNED_CLOSE)
+			return -1;
+
 		if (lws_process_ws_upgrade2(wsi->parent))
 			return -1;