barriers: Don't allow destroying other client's barriers

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jasper St. Pierre <jstpierre@mecheye.net>

Xi/xibarriers.c

@@ -534,6 +534,9 @@ XIDestroyPointerBarrier(ClientPtr client,
         return err;
     }
 
+    if (CLIENT_ID(stuff->barrier) != client->index)
+        return BadAccess;
+
     FreeResource(stuff->barrier, RT_NONE);
     return Success;
 }