bluray: fix abort() when empty overlay is closed

HDMV titles can construct empty overlay with invisible buttons to enable navigation.
This is used in ex. photo galleries (to flip image) and with audio navigation ("spoken menu").
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/access/bluray.c

@@ -791,10 +791,12 @@ static void blurayCloseOverlay(demux_t *p_demux, int plane)
             return;
 
     /* All overlays have been closed */
-    var_DelCallback(p_sys->p_vout, "mouse-moved", onMouseEvent, p_demux);
-    var_DelCallback(p_sys->p_vout, "mouse-clicked", onMouseEvent, p_demux);
-    vlc_object_release(p_sys->p_vout);
-    p_sys->p_vout = NULL;
+    if (p_sys->p_vout != NULL) {
+        var_DelCallback(p_sys->p_vout, "mouse-moved", onMouseEvent, p_demux);
+        var_DelCallback(p_sys->p_vout, "mouse-clicked", onMouseEvent, p_demux);
+        vlc_object_release(p_sys->p_vout);
+        p_sys->p_vout = NULL;
+    }
 }
 
 /*