newaccountsettings: fix link device window

Previously, we used the pointer to the text of the entry. But this
pointer is rewritten before we use it. So copy it into a string
before.

Change-Id: I9316fe7377a781dab2be37b281ee74be2195a936
Gitlab: #860
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>