Fixed a vlc_mutex_t leak in direct3d video output.

(cherry picked from commit d61f3b7fc6d02a117c53d89e74e42dbe809b697a)
Signed-off-by: Rémi Duraffort <ivoire@videolan.org>

modules/video_output/msw/direct3d.c

@@ -216,6 +216,7 @@ static void Close(vlc_object_t *object)
     vout_display_t * vd = (vout_display_t *)object;
 
     var_DelCallback(vd, "direct3d-desktop", DesktopCallback, NULL);
+    vlc_mutex_destroy(&vd->sys->lock);
 
     Direct3DClose(vd);