gnrc_sixlowpan_frag_vrb: set src_len to 0 on remove
gnrc_sixlowpan_frag_vrb: set src_len to 0 on remove

45f7966 made the `src_len` field the "emptiness signifier" for the VRB.
However, when `gnrc_sixlowpan_frag` is compiled in, the remove function
`gnrc_sixlowpan_frag_vrb_rm()` does not set the `src_len` to zero,
resulting in already deleted entry to be recognized as non-empty.

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