tevent: Clarify apparently useless conditions

tevent: Clarify apparently useless conditions

Comment on two similar conditions in tevent_standard.c, which,
otherwise, at a first glance, seem useless, i.e. always true.

The conditions checking glue->epoll_ops for being non-NULL, imply that
it *can* be NULL. A casual reader would not generally expect a "member"
function to modify its container's pointer in a container higher up, and
would assume that glue->epoll_ops could be NULL before the call,
resulting in a near-NULL pointer dereference.

However, in this case epoll_ops is indeed cleared in those "member"
functions, in the case of an epoll interface failure, to signify
fallback to poll interface.

Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Uri Simchoni <uri@samba.org>
Autobuild-User(master): Uri Simchoni <uri@samba.org>
Autobuild-Date(master): Tue Jul 12 13:56:41 CEST 2016 on sn-devel-144