Fix mca_coll_basic_alltoallw_inter()

Avoid sending/receiving zero size messages in order to be compliant
with the top-level modification

cmr=v1.8.2:ticket=4651:reviewer=bosilca

This commit was SVN r31836.

The following Trac tickets were found above:
  Ticket 4651 --> https://svn.open-mpi.org/trac/ompi/ticket/4651

ompi/mca/coll/basic/coll_basic_alltoallw.c

@@ -290,15 +290,23 @@ mca_coll_basic_alltoallw_inter(void *sbuf, int *scounts, int *sdisps,
     size = ompi_comm_remote_size(comm);
 
     /* Initiate all send/recv to/from others. */
-    nreqs = size * 2;
+    nreqs = 0;
     preq = basic_module->mccb_reqs;
 
     /* Post all receives first -- a simple optimization */
     for (i = 0; i < size; ++i) {
+        size_t msg_size;
+        ompi_datatype_type_size(rdtypes[i], &msg_size);
+        msg_size *= rcounts[i];
+       
+        if (0 == msg_size)
+            continue;
+
         prcv = ((char *) rbuf) + rdisps[i];
         err = MCA_PML_CALL(irecv_init(prcv, rcounts[i], rdtypes[i],
                                       i, MCA_COLL_BASE_TAG_ALLTOALLW,
                                       comm, preq++));
+        ++nreqs;
         if (OMPI_SUCCESS != err) {
             mca_coll_basic_free_reqs(basic_module->mccb_reqs,
                                      nreqs);
@@ -308,11 +316,19 @@ mca_coll_basic_alltoallw_inter(void *sbuf, int *scounts, int *sdisps,
 
     /* Now post all sends */
     for (i = 0; i < size; ++i) {
+        size_t msg_size;
+        ompi_datatype_type_size(sdtypes[i], &msg_size);
+        msg_size *= scounts[i];
+
+        if (0 == msg_size)
+            continue;
+
         psnd = ((char *) sbuf) + sdisps[i];
         err = MCA_PML_CALL(isend_init(psnd, scounts[i], sdtypes[i],
                                       i, MCA_COLL_BASE_TAG_ALLTOALLW,
                                       MCA_PML_BASE_SEND_STANDARD, comm,
                                       preq++));
+        ++nreqs;
         if (OMPI_SUCCESS != err) {
             mca_coll_basic_free_reqs(basic_module->mccb_reqs,
                                      nreqs);