coll/libnbc: fix NBC_Unpack()

always initialize 'size'.

Only the a2a_sched_diss() alltoall algorithm is impacted,
and this algo is currently unused, so there is no need
to backport nor update the NEWS file for now.

Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
(cherry picked from commit ff48e92)
Signed-off-by: Brian Barrett <bbarrett@amazon.com>

ompi/mca/coll/libnbc/nbc_internal.h

@@ -516,14 +516,18 @@ static inline int NBC_Unpack(void *src, int srccount, MPI_Datatype srctype, void
   int res;
   ptrdiff_t ext, lb;
 
+  res = ompi_datatype_pack_external_size("external32", srccount, srctype, &size);
+  if (OMPI_SUCCESS != res) {
+    NBC_Error ("MPI Error in ompi_datatype_pack_external_size() (%i)", res);
+    return res;
+  }
 #if OPAL_CUDA_SUPPORT
   if(NBC_Type_intrinsic(srctype) && !(opal_cuda_check_bufs((char *)tgt, (char *)src))) {
 #else
   if(NBC_Type_intrinsic(srctype)) {
 #endif /* OPAL_CUDA_SUPPORT */
     /* if we have the same types and they are contiguous (intrinsic
      * types are contiguous), we can just use a single memcpy */
-    res = ompi_datatype_pack_external_size("external32", srccount, srctype, &size);
     res = ompi_datatype_get_extent (srctype, &lb, &ext);
     if (OMPI_SUCCESS != res) {
       NBC_Error ("MPI Error in MPI_Type_extent() (%i)", res);