net/nanocoap: fix remaining space calculation
net/nanocoap: fix remaining space calculation

The space calculation must use the same length as the eventual
return value.

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