If handling an `NMActiveConnection` which is still activating (in state
`NM_ACTIVE_CONNECTION_STATE_ACTIVATING`), its `connection` property will
be `NULL`, rather than pointing to a valid `NMConnection`. Previously,
we were assuming that it would always point to a valid `NMConnection`,
and hence were crashing if not.

Handle that case by monitoring state and property change signals from
`NMActiveConnection` and connecting to the `connection` if it ever
becomes non-`NULL`. This will likely result in multiple connections to
the `NMConnection`, but that should be fine — it’ll only result in some
spurious emissions of `connection-details-changed`, and all the signal
connections should be dropped when the last reference to the
`NMConnection` is dropped. We don’t hold that reference.

This is easy to reproduce by having a VPN set up which takes a few
seconds to connect to. While it’s `ACTIVATING`, `mogwai-scheduled` can
attempt to query the connection information.

Signed-off-by: Philip Withnall <withnall@endlessm.com>