hashes/sha256: don't call memcpy if len==0
hashes/sha256: don't call memcpy if len==0

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