tests/lwip_sock_tcp: fix NDEBUG compile problem
tests/lwip_sock_tcp: fix NDEBUG compile problem

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