gnrc/pktbuf: avoid memcpy if size <= 0
gnrc/pktbuf: avoid memcpy if size <= 0

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