Fixes *alltoall* collectives at top level

This commit :
 - Correctly retrieve the communicator size when
   checking memory and parameters
 - Ensure (sendtype,sendcount) and (recvtype,recvcount)
   matches and return with MPI_ERR_TRUNCATE otherwise
 - Return with MPI_SUCCESS without invoking the low level
   if no data is going to be transferred
 - Fixes #4506

cmr=v1.8.2:reviewer=bosilca