_setup_settings(): fix initialization order dependency crash

`_generate_new_bitmessage_address()` depends on `self.guid`.

If the client runs first without `bitmessage` running, a crash will occur here because `self.guid` is absent.

fixes crash introduced in 5778d6cd436da6d05fcf12a0fc90171e8789f679
(`_setup_settings`: be more robust against partial db creation)