backend/x11: prevent segfault on empty DRI3 response

backend/x11/backend.c

@@ -448,7 +448,8 @@ struct wlr_backend *wlr_x11_backend_create(struct wl_display *display,
 			xcb_dri3_query_version(x11->xcb, 1, 2);
 		xcb_dri3_query_version_reply_t *dri3_reply =
 			xcb_dri3_query_version_reply(x11->xcb, dri3_cookie, NULL);
-		if (dri3_reply && dri3_reply->major_version >= 1) {
+		if (dri3_reply) {
+			if (dri3_reply->major_version >= 1) {
 				x11->have_dri3 = true;
 				x11->dri3_major_version = dri3_reply->major_version;
 				x11->dri3_minor_version = dri3_reply->minor_version;
@@ -458,6 +459,9 @@ struct wlr_backend *wlr_x11_backend_create(struct wl_display *display,
 					dri3_reply->major_version, dri3_reply->minor_version);
 			}
 			free(dri3_reply);
+		} else {
+			wlr_log(WLR_INFO, "X11 does not support required DRi3 version");
+		}
 	} else {
 		wlr_log(WLR_INFO, "X11 does not support DRI3 extension");
 	}