Any non-blocking P2P function should set the request to something else
then MPI_REQUEST_NULL, when passed a source/dest of MPI_PROC_NULL.
Introduce a new ompi_request_empty, which returns the
MPI_SOURCE=MPI_PROC_NULL, MPI_TAG=MPI_ANY_TAG, count=0 as specified by
the MPI-standard.
This commit was SVN r6611.