Fix issue #163 'coredump adding alias to chatty'

When starting Chatty for the first time, the SMS account wasn't instantiated. This is why an initially received SMS didn't close the welcome screen, and the attempt to send an SMS to a newly created contact led to a crash.

Should also fix https://source.puri.sm/Librem5/chatty/issues/166, since it is also caused by the attempt to establish a conversation without an instantiated purple_conv struct.