always initialize 'size'.
Only the a2a_sched_diss() alltoall algorithm is impacted,
and this algo is currently unused, so there is no need
to backport nor update the NEWS file for now.
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
(cherry picked from commit ff48e9286430b37aac3146efe2b355f255db94d5)
Signed-off-by: Brian Barrett <bbarrett@amazon.com>