roster: make more use of tp_clear_pointer
roster: make more use of tp_clear_pointer

jku/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,jku/telepathy-gabble