kerberos: Correctly set the error if object creation failed
kerberos: Correctly set the error if object creation failed

The error is actually a GError **, and not a GError *. There is no need
to set it to NULL.

Fixes: https://bugzilla.gnome.org/727896

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