tests/thread_msg_block_race: fix NDEBUG compile problem
tests/thread_msg_block_race: fix NDEBUG compile problem

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