nbc ireduce_scatter_block: ensure to check the correct return code

This was CID 709594 and 709592

ompi/mca/coll/libnbc/coll_libnbc_ireduce_scatter_block.c

@@ -52,7 +52,7 @@ int ompi_coll_libnbc_ireduce_scatter_block(void* sendbuf, void* recvbuf, int rec
   if (MPI_SUCCESS != res) { printf("MPI Error in MPI_Comm_rank() (%i)\n", res); return res; }
   res = MPI_Comm_size(comm, &p);
   if (MPI_SUCCESS != res) { printf("MPI Error in MPI_Comm_size() (%i)\n", res); return res; }
-  MPI_Type_extent(datatype, &ext);
+  res = MPI_Type_extent(datatype, &ext);
   if (MPI_SUCCESS != res) { printf("MPI Error in MPI_Type_extent() (%i)\n", res); return res; }
   
   schedule = (NBC_Schedule*)malloc(sizeof(NBC_Schedule));