typedef void (*SilcFSMDestructor)(SilcFSM fsm, void *fsm_context,
                                      void *destructor_context);

    The destructor callback that was set in silc_fsm_alloc or in
    silc_fsm_init function.  It will be called when a state function
    returns SILC_FSM_FINISH.  This function will be called through
    the scheduler; it will not be called immediately after the state
    function returns SILC_FSM_FINISH, but will be called later.  The
    `fsm' can be freed in this function.