kerberos: Plug leak in error handling

It's a little tedious to convert a kerberos error into a GError,
so there's a helper function to do it: set_error_from_krb5_error_code

That helper function leaks the format string use for constructing the
GError's message.

This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=739590