server: Avoid a crash if a registry notify event has been set already.
diff --git a/server/registry.c b/server/registry.c
index f2a5ce5..b0c815d 100644
--- a/server/registry.c
+++ b/server/registry.c
@@ -1954,7 +1954,8 @@
             notify = find_notify( key, current->process, req->hkey );
             if (notify)
             {
-                release_object( notify->event );
+                if (notify->event)
+                    release_object( notify->event );
                 grab_object( event );
                 notify->event = event;
             }