net/mqttsn: add min&max allowed len for client ID
net/mqttsn: add min&max allowed len for client ID

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