drivers/mtd_mapper: fix NDEBUG compile problem
drivers/mtd_mapper: fix NDEBUG compile problem


yogo1212/RIOT,kaspar030/RIOT,OTAkeys/RIOT,yogo1212/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,basilfx/RIOT,jasonatran/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,authmillenon/RIOT,jasonatran/RIOT,miri64/RIOT,basilfx/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,authmillenon/RIOT,jasonatran/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,basilfx/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,OlegHahm/RIOT,yogo1212/RIOT,kaspar030/RIOT,OTAkeys/RIOT