IPA/AD: check auth ctx before using it
    
    In e6b6b9fa79c67d7d2698bc7e33d2e2f6bb53d483 a feature was introduced to
    set the 'canonicalize' option in the system-wide Kerberos configuration
    according to the settings in SSSD if the AD or IPA provider were used.
    Unfortunately the patch implied that the auth provider is the same as
    the id provider which might not always be the case. A different auth
    provider caused a crash in the backend which is fixed by this patch.
    
    Resolves https://fedorahosted.org/sssd/ticket/3234
    
    Reviewed-by: Petr Cech <pcech@redhat.com>
    (cherry picked from commit ea11ed3ea6291488dd762033246edc4ce3951aeb)