gnrc_pktbuf_malloc: allow write-protect of size 0 snips
gnrc_pktbuf_malloc: allow write-protect of size 0 snips

Size 0 snips are legal packet snips (empty payload e.g.) so it doesn't
make sense to issue an error in the write-protection in that case.

API documentation doesn't mention it either and the tests still pass
with the check removed.

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