It is a mistake when refactoring the API to use frame cursors. 9853cbdd

All functions related to stack manipulation should take a frame cursor as a parameter. Therefore,

    void        (*push_frame   )(MuCtx *ctx, MuStackRefValue stack, MuFuncRefValue func);

should be

    void        (*push_frame   )(MuCtx *ctx, MuFCRefValue cursor, MuFuncRefValue func);