poll(2) and ppoll(2) implementations need to check and write to
.revents on the initial scan, anyways.  So any poll/ppoll call
which returns a positive result can be expected to have an
initialized .revents value.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65553 b2dd03c8-39d4-4d8f-98ff-823fe69b080e