gnrc_ipv6_nib: release packet when NC entry can't be added
gnrc_ipv6_nib: release packet when NC entry can't be added

When the neighbor cache is full, the packet trying to resolve a
neighbor is not released.

This change fixes that.

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