sss_autofs: Do not try to free empty autofs context
    
    If initialisation fails in function _sss_setautomntent, context will not be
    initialized and automount client will crash.
    The function _sss_endautomntent should not try to dereference NULL pointer.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2288
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 01f0d708fddfd50bdbb36a63737dcdc2c2fbd28d)