kerberos: Don't complete and unref the async operation yet
kerberos: Don't complete and unref the async operation yet

Another call to look_up_identity is in progress, so the operation
would be complete in on_identity_looked_up.

Fixes: https://bugzilla.gnome.org/688018

Distrotech/gnome-online-accounts,ritzk/gnome-online-accounts,Distrotech/gnome-online-accounts,halfline/gnome-online-accounts,felipeborges/gnome-online-accounts,Distrotech/gnome-online-accounts,felipeborges/gnome-online-accounts,dnohales/gnome-online-accounts,dnohales/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,halfline/gnome-online-accounts,felipeborges/gnome-online-accounts,dnohales/gnome-online-accounts