typerep/yaksa: fix use of builtin_element_size

If the datatype is from a flatted type, builtin_element_size can be
garbage because the field is not copied in struct flatten_hdr.

Obtain the builtin_element_size from basic_type instead.

src/mpi/datatype/typerep/src/typerep_yaksa_pack.c

@@ -142,7 +142,10 @@ static int typerep_do_pack(const void *inbuf, MPI_Aint incount, MPI_Datatype dat
         MPIR_Datatype *dtp;
         MPIR_Datatype_get_ptr(datatype, dtp);
         is_contig = dtp->is_contig;
-        element_size = dtp->builtin_element_size;
+        /* NOTE: dtp->element_size may not be set if dtp is from a flattened type */
+        if (dtp->basic_type != MPI_DATATYPE_NULL) {
+            element_size = MPIR_Datatype_get_basic_size(datatype);
+        }
         inbuf_ptr = MPIR_get_contig_ptr(inbuf, dtp->true_lb);
         total_size = incount * dtp->size;
     }