dbus: fix racy track addition (refs #17451)
dbus: fix racy track addition (refs #17451)

This fixes potential assertion failures. Insertion position must always
be valid.

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc