kerberos: Fail early on ticket request when ticketing disabled
kerberos: Fail early on ticket request when ticketing disabled

Right now, it's possible to crash gnome-online-account if you
turn of the kerberos slider at just the right moment when it's
doing a refresh.

This is because it will try to get a ticket when ticketing has
already been disabled (and the ticketing interface is unset).

This commit adds a check for that.

https://bugzilla.gnome.org/show_bug.cgi?id=756498

ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts