presence: use tp_strdiff instead of strcmp to be NULL-safe
presence: use tp_strdiff instead of strcmp to be NULL-safe

If we get the capabilities of a bare jid then the resource=NULL and
when finding the appropriate Resource* then it will crash as strcmp
does not handle NULL char*s well.

Signed-off-by: Jonny Lamb <3d1fb3642aaab2ff70e3392a5ab916bf99eed6a3@collabora.co.uk>

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