socket_zep: fix undefined timestamp conversion behavior
socket_zep: fix undefined timestamp conversion behavior

basilfx/RIOT,A-Paul/RIOT,yogo1212/RIOT,toonst/RIOT,basilfx/RIOT,mtausig/RIOT,jasonatran/RIOT,rfuentess/RIOT,ant9000/RIOT,josephnoir/RIOT,lazytech-org/RIOT,cladmi/RIOT,mtausig/RIOT,lazytech-org/RIOT,jasonatran/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,kYc0o/RIOT,smlng/RIOT,smlng/RIOT,kaspar030/RIOT,cladmi/RIOT,mfrey/RIOT,mfrey/RIOT,josephnoir/RIOT,miri64/RIOT,miri64/RIOT,OlegHahm/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,mtausig/RIOT,aeneby/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,aeneby/RIOT,rfuentess/RIOT,A-Paul/RIOT,OlegHahm/RIOT,aeneby/RIOT,x3ro/RIOT,A-Paul/RIOT,smlng/RIOT,OTAkeys/RIOT,x3ro/RIOT,ant9000/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,rfuentess/RIOT,BytesGalore/RIOT,rfuentess/RIOT,josephnoir/RIOT,mtausig/RIOT,toonst/RIOT,authmillenon/RIOT,A-Paul/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,x3ro/RIOT,basilfx/RIOT,RIOT-OS/RIOT,toonst/RIOT,BytesGalore/RIOT,mfrey/RIOT,BytesGalore/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,yogo1212/RIOT,kYc0o/RIOT,jasonatran/RIOT,authmillenon/RIOT,cladmi/RIOT,kaspar030/RIOT,authmillenon/RIOT,BytesGalore/RIOT,cladmi/RIOT,cladmi/RIOT,toonst/RIOT,BytesGalore/RIOT,lazytech-org/RIOT,toonst/RIOT,ant9000/RIOT,mfrey/RIOT,lazytech-org/RIOT,x3ro/RIOT,smlng/RIOT,aeneby/RIOT,kaspar030/RIOT,yogo1212/RIOT,x3ro/RIOT,josephnoir/RIOT,smlng/RIOT,yogo1212/RIOT,yogo1212/RIOT,mfrey/RIOT,josephnoir/RIOT,A-Paul/RIOT,miri64/RIOT,basilfx/RIOT,rfuentess/RIOT,kaspar030/RIOT,basilfx/RIOT,OlegHahm/RIOT,ant9000/RIOT,OTAkeys/RIOT,aeneby/RIOT,RIOT-OS/RIOT,mtausig/RIOT