Title: #272: cache_req: Fix warning maybe-uninitialized
While this patch will certainly fix the compiler warning I wonder if it wouldn't be
better to call talloc_new() after req is set in 'req =
tevent_req_callback_data(subreq, struct tevent_req);'. This way talloc_req_error() is
called with a proper req instead of NULL?
See the full comment at https://github.com/SSSD/sssd/pull/272#issuecomment-301431639