madmpi: fix MPI_*recv with MPI_PROC_NULL
MPI 4.0 states A receive from MPI_PROC_NULL succeeds and returns as soon as possible with no modifications to the receive buffer. (bottom of page 101).
It is already correctly managed for MPI_*send.
Prochaines maintenances programmées: lundi 06/05, lundi 03/06, lundi 01/07
Pour plus d'informations: https://doc-si.inria.fr/display/SU/Gitlab
MPI 4.0 states A receive from MPI_PROC_NULL succeeds and returns as soon as possible with no modifications to the receive buffer. (bottom of page 101).
It is already correctly managed for MPI_*send.