Merged zarf's changes, fix erroneous free().

- Merged non-NULL z_mem verification in fizmo_new_screen_size().
- Merged non-NULL z_mem check in write_interpreter_info_into_header().
- Merged max-undo-steps evaluation and z_mem-NULL-reset in fizmo_start().
- Fixed erroneous xdg_config_home-NULL-check in fizmo_start().