_setup_settings(): fix initialization order dependency crash

_generate_new_bitmessage_address() depends on self.guid

if the client runs first without bitmessage running, then next
with bitmessage running, a crash would occur here because no self.guid

fixes crash introduced in 5778d6cd436da6d05fcf12a0fc90171e8789f679
(_setup_settings: be more robust against partial db creation)