tp_mixin_(instance|class)_get_offset: return 0 without error if there is no mixin
tp_mixin_(instance|class)_get_offset: return 0 without error if there is no mixin

There's a critical warning in tp_mixin_offset_cast which will still be
triggered if you use (for instance) TP_GROUP_MIXIN on an unsuitable object,
and having the get_offset functions not error means they can be used to
implement "does this object have the mixin?"

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib