loginDialog: fix cancel from "Not Listed?"

Since commit 1ae0fadb we
no longer start a PAM conversation as soon as the user clicks
the "Not Listed?" button.  Instead we defer starting the PAM
conversation until the user types their username.

Unfortunately, the cancel button resets the dialog back to the
user list indrectly by cancelling the current PAM conversation.

This means if the user hasn't yet entered thier username then
the cancel button doesn't work.

This commit performs a direct dialog reset in the case the
PAM conversation hasn't been started yet.

https://bugzilla.gnome.org/show_bug.cgi?id=693756