tp_proxy_poll_features: move checking for invalidation inside the loop
tp_proxy_poll_features: move checking for invalidation inside the loop

We call out to user code from the loop, which could conceivably invalidate
us.

Also hold a ref for the duration of the loop, for the same reason.

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