correctly handle the case in which iovec is full and the
last accessed element of the datatype is the beginning of a loop
Refs. open-mpi/ompi#6285
Thanks Axel Huebl for reporting this
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
(back-ported from commit open-mpi/ompi@0832ab5acc)