NSS: Use enum_ctx as memory_context in _setnetgrent_set_timeout()
    
    We've noticed some crashes that happened because enum_ctx is already
    freed, but the timeout handler is still called. In order to avoid that,
    let's remove the timeout handler when enum_ctx is freed at other places.
    
    Resolves: https://pagure.io/SSSD/sssd/issue/3523
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>