sys/cb_mux/cb_mux.c: fix cpp.check unitialized variable
sys/cb_mux/cb_mux.c: fix cpp.check unitialized variable

yogo1212/RIOT,kaspar030/RIOT,kYc0o/RIOT,josephnoir/RIOT,aeneby/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,x3ro/RIOT,mtausig/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,josephnoir/RIOT,aeneby/RIOT,ant9000/RIOT,ant9000/RIOT,mtausig/RIOT,authmillenon/RIOT,x3ro/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,ant9000/RIOT,mtausig/RIOT,josephnoir/RIOT,authmillenon/RIOT,toonst/RIOT,OTAkeys/RIOT,josephnoir/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,toonst/RIOT,jasonatran/RIOT,miri64/RIOT,toonst/RIOT,mtausig/RIOT,kaspar030/RIOT,basilfx/RIOT,aeneby/RIOT,basilfx/RIOT,kYc0o/RIOT,x3ro/RIOT,kaspar030/RIOT,aeneby/RIOT,miri64/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,smlng/RIOT,jasonatran/RIOT,x3ro/RIOT,smlng/RIOT,RIOT-OS/RIOT,smlng/RIOT,authmillenon/RIOT,aeneby/RIOT,kYc0o/RIOT,mtausig/RIOT,toonst/RIOT,x3ro/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,ant9000/RIOT,basilfx/RIOT,authmillenon/RIOT,jasonatran/RIOT,smlng/RIOT,jasonatran/RIOT,yogo1212/RIOT,toonst/RIOT,miri64/RIOT,jasonatran/RIOT,basilfx/RIOT,josephnoir/RIOT,OTAkeys/RIOT,yogo1212/RIOT,yogo1212/RIOT,kYc0o/RIOT,authmillenon/RIOT,smlng/RIOT,miri64/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,miri64/RIOT,OlegHahm/RIOT