xwayland: make sure client is not gone in sync callback

in XWayland, dri3_send_open_reply() is called from a sync callback, so
there is a possibility that the client might be gone when we get to the
callback eventually, which leads to a crash in _XSERVTransSendFd() from
WriteFdToClient() .

Check if clientGone has been set in the sync callback handler to avoid
this.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99149
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100040
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1416553
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Tested-by: Mark B <mark.blakeney@bullet-systems.net>

hw/xwayland/xwayland-glamor.c

@@ -435,9 +435,12 @@ static void
 sync_callback(void *data, struct wl_callback *callback, uint32_t serial)
 {
     struct xwl_auth_state *state = data;
+    ClientPtr client = state->client;
 
-    dri3_send_open_reply(state->client, state->fd);
-    AttendClient(state->client);
+    if (!client->clientGone) {
+        dri3_send_open_reply(client, state->fd);
+        AttendClient(client);
+    }
     free(state);
     wl_callback_destroy(callback);
 }