KRB5: Do not switch_creds() if already the specified user
The code didn't have to handle this case previously as sssd_be was always
running as root and switching to the ccache as the user logging in.
Also handle NULL creds on restore_creds() in case there was no switch.
One less if-condition and fewer indentation levels.
Reviewed-by: Sumit Bose <firstname.lastname@example.org>
Reviewed-by: Lukáš Slebodník <email@example.com>